I don't think it can be truthfully said that Western nations did not have a problem sustaining immense casualties. Speaking from the Canadian perspective, the sheer number of men thrown into action was the subject of intense debate during the First World War. In fact, the 1917 election was basically all about it. The Borden Union Government was formed out of the conscription debate as a brilliant move by Borden to revive the factionally divided politics of the time.

...[C]onscription was the big issue; it acted as a solvent on Liberal Party bonds while it brought together an amazing array of political talent in the Union Government. It was the dominant issue before the electors at the polls. In all the provinces of Canada was sentimentalized and exaggerated; it was screamed from the hustings and thundered from the pulpits. Those against it were "cowards and traitors" and those for were "autocrats worse than the Kaiser." ^1

It was an incredibly divisive issue. There were many who openly and adamantly opposed it. Canadian Nationalists published a book called "The Case Against Conscription" in 1917, I believe that Henri Bourassa's sectionm "Win the War," And Lose Canada summarizes a lot of the sentiments that opponents of conscription felt. There were serious arguments over whether pouring more men into Europe was the most effective course of action. Bourassa's book puts particular emphasis on agriculture and a lack of obligation to the European continent. Even today there is historical argument over the merits of conscription policies.

So I do not think it is, as you put it, "a shift in values" (at least in the case of Canada). There has always been a certain aversion to the loss of life in Canadian politics.

^1 Willms, A. M. "Conscription, 1917: A Brief for the Defence." Canadian Historical Review 37, no. 4 (1956): 338-51. Accessed November 30, 2016. doi:10.3138/chr-037-04-02.

As a bit of a follow-up question and a possible answer, what was the reporting of the time like?

Today if 1 soldier dies in Afghanistan it will take up a few seconds of the evening news. During WW1/2 were newspapers and radio programs releasing fatality statistics on a regular basis?

Let's go back to 1914. I'll talk a bit more about Germany because it's the one I can quote more about.

Just a few decades earlier Germany had achieved unification and had defeated France at war (1870-1871). That war had been quick (less than a year), Germany under Wiliam II had improved its navy (in what was known as Weltpolitik), the Industrial Revolution ha brought more deadly weapons and nationalism was popular among Germans.

Come 1914, there was a consensus of support around the war: "The great majority of German's leading writers, artists, composers, and academic scholars were passionately engaged in support of the war effort." (Chickering, p. 132). In France the president, Raymond Poincaré, called for a "Unión Sacrée" (Holy Union), calling the different parties to forget its differences in order to protect France.

However the war proved to be far more deadly than expected. To put in context, 124 deaths per 1000 pre-war male population in Germany, 161 in France, 252 in Austria-Hungary. Russia lost more than 5000000 civilians during the war. (Chickering, 192. Disclaimer: the autor advises caution with these figures since collecting war statistics, he argues, was difficult). That's massive. There were mutinies among French divisions. Germany had to endure a war economy: food was rationed, prices were frozen, the cold weather of 1916 and 1917 destroyed potato crops, coal production went down (10% less 1917 than it was before the war).

After the war a sentiment of pacifism appeared and grew in popularity, specially among the winners (France, the U.K.). Already Russia had quit the war in 1917, the revolutionary government being opposed to it. This sentiment was called among French the "der des der" (la dernière des dernières, the last of the last wars). Paul Kennedy explains that there were already politicians talking about an organisation where States will meet and solve their differences: Robert Cecil in the UK or, of course, Woodrow Wilson. This would came into existence in what was the League of Nations (LoN).

Nonetheless the treaty of Versailles, which also gave birth to the LoN, was seen as a "diktat", as something enforced shamefully, in Germany (Italy wasn't too happy about it either). This, among other factors, lead into WWII. And then again, we need to remember that war was unpopular in both France and the UK before WWII: the Front Populaire (France's left coalition that ruled between 1936 and 1938) refused to send troops to Spain during the Civil War, Neville Chamberlain returned celebrating to the UK after the Munich conference saying how he had Hitler's promise to promote peace. Governments were already "allergic", to say it this way", to war.

Did the Vietnam war play a role in this?

Why there is less war nowadays, specially among Great Powers, than before may be a question better suited for r/AskSocialScience. The Vietnam war played a role on this: it spawned massive protests. The euro-missiles crisis of 1983 also shows that opposition to war is important.

TL;DR: In conclusion, although this topic would benefit of a more extended study, the lethality of WWI and the costs it provoked had already created an aversion fo governments and people towards war.

Sources: George Chickering (2004), Imperial Germany and the Great War, 1914-1918. Second Edition, Cambridge University Press.

Michael Sturmer (2000), The German Empire, 1870-1918, The Modern Library.

Paul Kennedy (2006), The Parliament of Man, Random House Mondadori.

Isn't it a reasonable statement that the public's perceived importance of the war affects its expected costs (not at all specifically quantified)? As you already allude to regarding World War 2, when the stakes are higher a higher cost is expected.

I know what you're getting at in your question, but you also seem to gloss over that very important piece of context you yourself mentioned. Would you consider any of the conflicts fought by "Western" nations since World War 2 to have stakes anywhere close to as high as that war?

Intuitively this distinction should be very clear. For example, if the United Kingdom was, in 1989, to become embroiled in a conventional invasion of Western Europe by the Soviet Union, the public would view this as a matter of imminent importance to the state and therefore would be willing to accept much higher gross casualty numbers, right?

World War I and World War II could arguably be placed in similar context as the above hypothetical situation; to simplify, the state went to war to prevent European domination by a power unfriendly to Britain. The stakes were higher, and, as the wars grew longer, continued to become higher as the hypothetical cost of defeat became ever greater (the longer a country is at war and the more resources it expends, it may demand greater restitution from the defeated).

Whereas, if the operation was, say, defending peacekeeping efforts and targeting warlord forces in a fragmented distant third-world country like Somalia, the sudden and shocking loss of 19 soldiers would not be viewed as an acceptable cost. The general ambivalence of the public to such a cause meant that they were extremely reluctant to pay any cost, even a "low" (no disrespect to the fallen) one like 19 lives.

For an older example of that phenomenon, we can look at the Boer War. Fought from 1899 to 1902 in South Africa, it was a war fought against the Transvaal republics, ending in their eventual incorporation into South Africa. There were about 22,000 dead from that war, much lower than the millions of the later World War 1, but because of the nature of the war, it was to a nontrivial degree unpopular in Britain and more so in the Commonwealth countries.

The public's discontent with Vietnam is a very popular topic and has been well-studied already. Some of the main issues were the implementation of the draft and the perceived irrelevance of South Vietnam to the United States' interests. The discontent only grew as losses mounted because the costs had already far surpassed the "budgeted" expectations of the conflict.

We can't really tell for sure if Western standards have changed -- we simply have not fought a war even approaching the stakes of World War 2. Since we can't really come to any conclusion due to great contextual differences between the compared conflicts, we also can't rightly say there has a been a shift in values.

And, aside from the great variance of objective of conflict, there are also expectations derived from the public's perception of "power" or "technology". The Boer republics, Vietnam, and Iraq were (intuitively) not viewed as able to present the same resistance as, say, Germany or Russia.

This may be begging the question. Are we sure Western nations are as casualty averse as you are assuming? I wonder if it only seems so because of the smaller scale of the conflicts we have seen since 1945. This hesitance to sustain heavy losses may well evaporate in the event of a WWIII.

As follow up questions:

Around those same periods, we saw a shift away from relying on draft-style recruiting. Why?

And what role, if any, did the above have on the OP's question?

I think you are asking us to compare apples to oranges. WWII (and WWI to a lesser degree) posed an existential threat to the very sovereignty of most of the nations involved.

One of the reasons the American public withdrew their support of the Vietnam war was it was sold as an existential threat but quickly revealed itself to be a proxy war.

Defending the very nation you are from is much different from defending your 'national interests'.

I'm not able or qualified to answer this from a military perspective, but from a social perspective. Our attitudes about the preciousness of life has changed drastically over the past 100 years. Just over 100 years ago children died with far greater frequency than they do today. They died from work related accidents on farmers and in mines and factories. They died from many communicable diseases that we vaccinate or treat with penicillin today. As a consequence, families tended to be much larger. So if you had to send a son off to war to "defend your country," that was more of an acceptable loss. Now, so many families have fewer children, and even fewer sons, that every lost life makes a huge impact on that family. I'm not saying that people didn't love their children or grieve their loss, they certainly did. But young people's deaths weren't as shockingly rare and unusual as they are today.

Can't underestimate the impact of smaller families. Harder to put a son (or daughter) at risk when you only have two. The "greatest generation" had larger families, and many had lost siblings early in life too. North Atlantic Peoples (white folks) aren't what they used to be. Others have noticed and at some time in the future will, I believe, test our willingness to defend our autonomy. History is full of this.