Break ups happen for many different reasons. Each relationship is different, some things hurt people more than other people, some people get annoyed easier, some people want space, some people don't, some people cheat, some people don't, some people lie...you get the point.
There aren't certain written out reasons as to why people break up, things change.
To love is to suffer. To avoid suffering, one must not love; but then one suffers from not loving. Therefore, to love is to suffer, not to love is to suffer, to suffer is to suffer. To be happy is to love; to be happy then is to suffer, but suffering makes one unhappy; therefore to be unhappy one must love or love to suffer or suffer from too much happiness. I hope you're getting this down.