The most common reasons that people went on crusades were following the Pope's call, to be forgiven for past sins, and to loot and kill, according to the BBC. Seemingly less common reasons were proving one's bravery and seeking adventure, and also seeking foreign lands to own.
After the Muslim Turks defeated the army of the Byzantine Empire and settled in the holy lands in 1071, the Emperor of Byzantium asked Pope Urban II for help in winning them back. The Pope then appealed to the knights of Europe to go on a crusade to win back Jerusalem. Knights who had committed grave sins in the past were told by the Church that they could have their past sins forgiven by participating in this Holy War. The more violent knights of the time were encouraged to take part because they could kill as much as they wanted as long as their victim was Muslim, and this was sanctioned by the Pope and the Catholic Church. Lesser noblemen and landless men saw the crusades as an opportunity to both win a name for themselves and vastly increase their wealth as well as giving access to lands which could be seized by or awarded to a soldier for valor in battle.