I know Spain was technically neutral during both wars (although they did send supplies to the axis powers during WWII due to their support for the Nationalists during their civil war), what were the main reasons for its neutrality? I'm sure the Spanish Civil War directly before WWII created reluctance for Spain to enter another war. But was that the main reason? And before WWI did the country just not have strong enough alliances with any of the involved major powers? It seems like the country would have been heavily lobbied to join the war by various countries involved once troop casualties started to skyrocket.
For WWI Spain was already at that point a declined power; the 19th century was essentially the fall of Spain mainly starting in the Napoleonic Wars and concluding with the American War. Mexico in 1821, 1822 for Venezuela, Ecuador, and Colombia, and Peru in 1824. By 1818 the Chileans and Argentinians had de facto pushed the Spanish out of South-South America and 1821 they were fully pushed out. It took a few decades for them to be recognized but conclusively, by 1850, Spain had lost its entire mainland colonial holding in the Americas but its Caribbean holdings would also experience quite a number of revolts through this time in the revolutionary fervor. By 1898 Spain was conclusively out of the picture and was, by all measurements, no longer a colonial power with the loss of Guam, Puerto Rico, Cuba, and the Philippines to the Americans.
It was racked with war in the Napoleonic Wars, lost its massive colonies in the 1810's and 1820's, and lost whatever it had left in global influence/projection in 1898 to America. It was dirt freaking poor, without anything resembling a modern navy or army, and was just scraping by. Basically no one really wanted them. Whatever hope they had to join in on the fray last minute was killed in 1909 with what is called the Second Melillan campaign; it was an absolute disaster. They had no professional troops and basically conscripted a bunch of poor laborers (all the rich people bought off their conscription) and threw them into North Africa to quell a revolt. They were completely bashed in their offensive against 'easy' colonial natives and had to conscript more men and essentially pound them down with superior numbers to stop them. Through this revolts would spread across Spain like wildfire exacerbated by the failure of the campaign, economic downturns, and so on.
Essentially the line of reasoning was this: Spain lost all of her colonies and thus her wealth. What little glimmer of hope she had to be considered a Great Power through her extensive island possessions and force projection was lost when they were embarrassed by the Americans; the Spanish Empire was recognized to have been a facade at that point and the Americans exposed them and put the final nail in the coffin. Being at most a second rate power she would be reduced below even that when she couldn't even handle a bunch of untrained, loosely organized native peasantry in a colony; how the heck were they supposed to stand up to a Prussian or a French battalion? They'd be wiped out without contest. They were a liability at best.
Most reason of all though Spain benefited from trading with all sides in the war. A loose paraphrase I like is that it's said the United States won WWI as it entered the war as a beefy regional power and exited as a world power. They benefited, quite clearly, the most of the war. Spain and other neutral powers like that come in second. Their economy experienced a boom by standing on the side lines selling steel and fruits in particular to both sides.
TL;DR: Divided populace, shitty military showmanship in 1898 and 1909/10, poor and no more global presence, nothing resembling a modern military force and really no reason to go to war.
I hate to extend beyond the TL;DR but this is a great place to put it: Why would they? France wanted Alsace-Lorraine and to contain Germany, Russia wanted to protect Serbia, Britain wanted to contain Germany and protect Belgium, Italy wanted Austro-Hungarian lands (especially Tyrol and Trieste)...what beef does Spain have with Germany or Austria-Hungary? What land would it take from them or did it have any claim to? What did it have to gain from going to war with the Ottomans later on even? They'd be on the winning side but at what cost when they could just sit it out? Opposite is even worse...I imagine they'd go for Gibraltar or Southern France but they'd be molly-whooped. The Brits would have had absolutely zero issue enforcing a full embargo on Spain as well as well as maintaining defense at Gibraltar. Invasions from Gibraltar, France, and likely Portugal would topple their government in a measurably short amount of time.
TL;DR Part 2: Electric Boogaloo: War with Entente was winning less economically than staying neutral with no benefit of territorial gain and wouldn't be able to contribute enough to gain financial reparation. War with Central Powers meant land grabs against her, total and quick occupation, further economic collapse, total embarrassment, etc. Just total destruction.
For WWII I'd recommend this fantastic post by /u/Bernadito.
TL;DR: Franco kept asking for unreasonable demands in return for joining the Germans + Mussolini didn't want a Mediterranean competitor. He basically just kept pushing his limits with Hitler and Mussolini.
Don't forget the Blue Division, they were spanish facist volunteers who when to fight with the Germans in Russia. Their Banner made it back, I am not sure how many of them made it back. (I saw this in Madrid Military museum)
Re WWII, there are a couple of posts focusing on Spain in the FAQ