Many relationships fail because one party tries to overpower another,or demands too much. People in love tend to think that love will conquer all and their spouses will change the bad habits after marriage. Actually, this is not the case. There is a Chinese saying which carries the meaning that "It is easier to reshape a mountain or a river than a person's character."
It is not easy to change. Thus, having high expectation on changing the spouse character will cause disappointment and unpleasantness.