People originally joined the Crusades for the promise of an eternal place in Heaven or because their services were volunteered by lords. Once word of wealth and spoils began to spread, the Crusades became a treasure hunt for some participants.
The First Crusade was a mission to the holy land to defend Christians who, it was rumored, were being tortured in the Middle East. Once the first crusaders realized the amount of wealth the Islamic people of the East possessed and began returning with spoils, others decided to join so they too would have an opportunity to improve their financial situations by capturing a few of the riches, which were not taxed if taken in the name of God during a crusade.
In some situations, men did not join the Crusades for religious reasons or the prospect of finding riches but because they were offered land back home in return for their service. Jerusalem became somewhat of a Mecca for Christians after control of it was taken from the Muslims in 1076. As with the soldiers who first joined the Crusades for the promise of rewards in Heaven, Christians felts that a journey to Jerusalem would make them closer to God.