One could understand why a lowly baron might eye a duchy or why a duke might covert a throne. But why do kings wage war? If you are a French nobleman in the late middle ages, your estate already provides you with plenty of food, entertainment, luxury, and the opportunity for a mistress or two or even a harem. Why wage war?
It's such a pain in the backside. Some rulers spent most of their lives on the road going from one campaign to the next. Always managing quarreling commanders, military logistics, arduous terrain and various diseases one might get in army camps. Then you endure the stress of desertions, mutiny, and the actual combat part where you and some of your brothers and cousins might die, or you might get captured.
Why? Why bother? Especially easily defensible countries like Spain or England. Why bother yourself in terrible military campaigns in the pursuit of land gains your dumb ass heirs will probably squander in the future, when you can just lay back in your comfy castle with some wench washing your feet, quelling a peasant rebellion every once in a while and managing court drama.
Because warfighting and military concerns are intimately connected to the basis of your social and economic power. The aristocracy became the aristocracy in part because they were the ones who could afford modern warfighting gear, like horses, armor, and weapons. The state (or whatever diminished medieval equivalent existed whenever and wherever you choose to specify) didn't furnish it, the aristocracy did. Warfighting was a reflection of ones masculinity, and masculinity was the basis of one's fitness for position in the hierarchy of peers. If your fitness was questioned - perhaps by one's refusal or disinterest in waging war and instead sitting at home eating grapes and getting your feet washed - then your position in the hierarchy was, too.
This is not to say any of this is simple and there weren't examples of men in a military peerage that didn't fight, but there were extremely strong cultural forces that encouraged men of privileged classes to bear what they considered the holy burden of military service to their monarch or state or people or country (or whatever other cultural construction motivated them), at risk of ostracism, and loss of their property and position.
I've written a bit more about this.
Concentrating on how the medieval formation of the second estate remained a potent social force even into the 18th century, here
and about the role of knights in medieval warfare, here
for more on how armies were raised and equipped, check out this post as well as this one, both about mercenaries and military equipment.
To understand why aristocrats would so frequently take part in warfare, it is important to understand that medieval/early modern Europe was a profoundly honour-based society. Here is how the historian William Ian Miller has put it:
‘Honour permeated every level of consciousness: how you thought about yourself and others, how you held your body, the expectations you could reasonably have and the demands you could make on others … It was your very being. For in an honour-based culture there is no self-respect independent of the respect of others.’”
An aristocrat's power did not primarily come from their monetary wealth, but rather their good reputation. Understanding this overriding concern with honour allows us to make sense of actions that would otherwise appear quite baffling to our modern sensibilities. For example, in 1529 the French nobleman Gabriel Martel killed his peer Robert de Signes because of certain "odious letters indicative of hatred." Nobility were constantly involved litigation, epistolary exchange, and various forms of vindicatory violence from damaging property to formal duels to repair and defend their honour.
With that in mind, you can get a better sense why nobility would expend considerable resources and risk death in warfare. Combat was an essential part of a male aristocrat's life and an obvious outlet in which one could display one's honour. To oversimplify things a little, when a king or powerful noble wanted to engage in warfare, he would call upon their social network of nobles, and appeal to their sense of honour to request their support. Not only would this be an opportunity for them to demonstrate their own martial prowess, but it allowed them to gain social capital which they could cash in later.
That being said, it is worth noting that as the military utility of the nobility waned in early modern period, other avenues of earning social capital opened up. Expanding states required expanding bureaucracies and nobles often maintained their position in royal clientage networks by holding these offices. By the eighteenth century, it became more possible for aristocrats to avoid a life of warfare and live off the revenues from their estates and offices while still maintaining their honour and social capital within clientage networks. Honour remained important, vindicatory duels continued well into the nineteenth century, and nobles still served as officers in militaries across Europe, but the martial lifestyle became less pronounced in aristocratic culture.
Sources and further reading:
Carroll, Stuart. Blood and Violence in Early Modern France. New York: Oxford University Press, 2006.
Dewald, Jonathan. The European Nobility, 1400-1800. New York: Cambridge University Press, 1996.
Miller, William Ian. Humiliation : and other essays on honor, social discomfort, and violence. Cornell University Press. 1993.
Neuschel, Kristen B. Word of Honor: Interpreting Noble Culture in Sixteenth-Century France. Ithaca and London: Cornell University Press, 1989.
van Nierop, Henk. The Nobility of Holland: From Knights to Regents, 1500-1650. Translated by Maarten Ultee. Cambridge: Cambridge University Press, 1993.