Well, I'm not entirely sure I'm understanding your question, but I'm interpreting it to mean "Why is it that some people feel that in order to feel good, they must have something to feel bad about?"

If this is what you meant, then I'd say that some people literally are afraid of being completely happy. Some people's lives have largely consisted of being unsatisfied and/or unhappy to the point that the person becomes comfortable with feeling unsatisfied/unhappy. They view that as normal. Change is difficult for everyone. Therefore, even though the person wants to be happy, a part of them is still afraid to let go of their old routine/"normal" life. So when they find themselves in a situation where nothing seems to be wrong, they will find something to complain about because that makes them feel more comfortable, because that is what they are accustomed to.