They fought because the Christian church promised that anyone who died whilst fighting in the crusade would go to heaven and not have to worry about facing the horrors of hell. As well as this, the Pope was worried about the spread of violence in western Europe. young men who were being trained for war were fighting each other; the Pope thought that the crusades might help the problem. Also the Kings people wanted to be loyal to him and not be punished by him. Hope I helped. :)