Why do people only think of the bad times after a break up?

Alot of the time, the one who got dumped feels rejected so they have to make themselves feel better about the situation by saying that it wasn't a good relationship anyways. Or people are hurt and dissappointed that it didn't work out. By rationalizing that it wasn't a good relationship anyways, then you won't feel so hurt or disappointed. Dissing on people specifically is immature, though.