To be forgiven for past sins. The Pope offered forgiveness for anyone who took part. This was important for knights who had killed many people in battle.
To see the world, have an adventure and prove their bravery.
To get land overseas. This was tempting for a younger son who would not inherit his father's lands.
Serfs, peasants who belonged to their lord, joined the Crusades because the Pope promised them their freedom if they went.
To gain wealth.
Kings encouraged troublesome knights to go on Crusade because it got them out of the country.