Why do some people complain about everything?

I hate whiners. But my own boyfriend whines like crazy. He leaves nothing left. Even a tiny ant is a matter to be complained about. My sis hates everyone and complains about their lives. She doesnt even have friends now anymore. People block her from Facebook. So why is it that these people complain so much? I had several talks with my boyfriend about this issue, but it seems that he cannot digest food if can't complain one day.