One thing I think you may have misunderstood is that for these medieval noblemen, military service was to a great extent the point. Anglo-Norman feudalism was a military aristocracy.

The barons and knights don’t own the land as such, not as we would now understand it. Instead, they hold rights and privileges over their lands (and if you go back far enough, to the ancestor who “started the line”, were often first GRANTED the rights over their lands) in exchange for performing certain duties and responsibilities. Going to war on their liege lord’s behalf is one of those unavoidable duties - you either come when called and bring a suitable retinue with you, or you pay a scutage/ecuage tax (over time this becomes more and more common) which will allow your liege to hire men who can replace you and your retinue.

If you do not fulfil your obligations, then your liege lord can take the privileges you hold away from you and give them to someone else. The word “felon” (as in, “very serious crime”) originally referred to someone who had breached the feudal contract between overlord and vassal in this way.

It’s not “I want to go to war”, in the way a modern rich boy might decide to do it. It’s much more of an obligation, like taxes.