I think part of the premise of the question is flawed, but to explain why we need to answer both who is doing the viewing and who suffered the losses.
To start with the losses: while total losses in World War II were higher than World War I, this is not true for all combatant countries. A number of countries suffered much higher military deaths in the First World War than in the Second:
| Country | 1914-1918 Military Deaths | 1939-1945 Military Deaths |
|---|---|---|
| Australia | 59,000 | 39,000 |
| Canada | 56,000 | 42,000 |
| Newfoundland | 1,200 | 1,100 |
| New Zealand | 16,700 | 11,700 |
| UK (Home Islands) | 744,000 | 384,000 |
| France | 1,357,000 | 210,000 |
| Italy | 460,000 | 319,000 |
The numbers I'm citing are not exact, and in fact I picked the lower estimates to make a point. Not only are the absolute number of military deaths higher for those countries in the First World War, but they are also higher in relative percentage terms, given that those countries had smaller populations than they would in 1939. France and Italy in particular are somewhat shocking in that the military deaths in the First World War outnumber the military and civilian deaths in the Second in those countries, despite larger swathes of both countries being active battlegrounds in the Second World War, and having large resistance movements that at times approached civil war-levels of violence between their respective countries' citizens.
In addition to the military deaths being larger in the above countries in the First World War, the experience of the war was very different. For countries like France, Italy, or Britain, the First World War was largely experienced by military aged men in military fronts either abroad, or in narrowly defined sectors of the country. Which is to say that deaths were both larger in number but also more concentrated, and also more removed from the rest of the population. In contrast, the Second World War was more "total" in the sense that civilian populations in those countries often experienced the war firsthand in aerial bombings, more widely spaced military campaigns, and military occupation (you could probably count the massive influx of American servicemen to all of those countries as part of that "occupation").
This also needs to the question of who is viewing the First World War as tragic and somber. And here I suspect that general idea is heavily shaped by popular memory of the Western Front in World War I, which (again) is shaped especially by British, British Imperial, and French experiences and perspectives and memory.
This is not necessarily how other countries would view the two world wars, let alone in relation to each other. In the former Soviet Union, for example, the Second World War is by far treated as the bigger historic memory, even today. This had both triumphant aspects as well as tragic and memorial aspects - every Soviet city had (and still has, for the most part) its war memorial and eternal flame to the Second World War dead. The First World War, in contrast, has almost been lost to memory, both popular and from an academic historical perspective. It was long overshadowed by the 1917 Revolutions and the subsequent Civil War (the Civil War saw much more widespread civilian suffering, but smaller military losses and mobilization compared to the 1914-1917 war), and at best was often treated as a prologue to the Revolution, the true world-historical event. This has only just begun to change among historians of Russian history in the past decade and a half or so.
It's also worth noting that Eastern European countries often have a different view of the First World War from the Second, in that the Second was often much more deadly and destructive (and tragic) than the First, which often if anything is remembered as having a decent outcome. Poland and the Baltic States, for example, established themselves as independent countries after 1918, while the period after 1945 saw annexation or occupation.
There are many other countries that also will remember the wars differently because they participated in one and not the other. Historic memory in Turkey will care more about the First World War than the Second. East Asia, while it did see important events in the First World War (the Battle of Tsingtao, the 21 Demands and May 4 Movement), clearly experienced the Second World War in a much more devastating way.
The one country I'm leaving for last is Germany, because it's really in a case of its own. Memorialization of the dead in the First World War was very much a feature of German public life after 1918, but it also became incredibly politicized - honoring the war dead often went hand in hand with the "Stab in the Back" myth (ie, that the heroic military had been betrayed by civilians at the homefront, with those treacherous forces often being associated with socialists and Jews). Public commemorations of First World War dead were in particular seized upon by the Nazi regime both as public displays of patriotism and as a means of justifying their military aggressions. Ultimately, the Second World War turned out to be far, far worse, in terms of the military dead, civilian dead, destruction to Germany itself, its division and occupation, and culpability in crimes against humanity. The Second World War thus became bound up with ideas of shame, guilt, heroic resistance by a few, and a need to learn the lessons of history. In contrast, the First World War became much more forgotten. If anything, it became tragic in a sense that it wasn't really clear why Germany had fought in that war at all, and that it had only started a cycle of destruction that would get worse.
In summary - it really varies. But in general, if we're talking about memory of the First World War, more often than not we're really thinking about the Western Front and the countries that participated in the fighting there. For many of them, the losses were indeed greater there than in the Second World War.