People break up for multiple reasons. There is no definite reason why people would break up because every situation and every relationship is different. I would say that all breakups stem from the relationship just not working anymore. Either they aren't satisfied with it, they know that the relationship is somehow hindering one or both partners, or they can no longer be with each other due to various reasons. No one wants to be in a relationship that isn't going to work out in the end so the logical thing to do is the end the relationship or break up.