Why did people go on crusade?

-To become martyrs, and automatically go to heaven.

-Those who were not first-born had opportunity to achieve glory in battle, plunder loot, maybe get some land of their own when becomin a vassals of one crusader kings.

-Knights were trained in swordsmanship, riding and other fighting skills, they were bored if there were no wars. They were like "im sucha good figher i have a diploma in Lyon's school of swordsmanship" but the market in France has nothing to offer for someone of my skills, i feel like im wasting my life and i gotta do something about it."

To recover the Holy Land from the Muslims.