The crusades also offered uneducated young men the chance to gain a step up in society. The crusades created a new class of men who in the service of a noblemen were able to transcend societies locked in concept of "who" they were and "what" they were to do in life. Like the seafarers a few hundred years later, these were the first men to leave their isolated villages for foreign horizons. Is it coincidence that the concept of an individual "I" also emerged at the same as the crusades?
It was also an outlet for european military 'energy'. The feudal system was beginning to breakdown as only the eldest male could inherit land & title. There were a lot of excess males running around -- Men at Arms -- looking to gain a military reputation. Directing them toward the infidel made a lot of practical sense.