Let's imagine a woman named Maria. She has no real friends, no long term projects; there is nothing she really cares about. She spends most of her time just trying to climb the social ladder and get as drunk as she possibly can.
As soon as you hear this story, you might be thinking that Maria's life sounds pretty hollow and meaningless, but let's focus for a moment on a simpler question. Forget about whether her life is a good one or a bad one, and just ask yourself: Could she possibly be happy?
According to many psychologists, the answer is obviously yes. These psychologists define happiness as simply being a certain kind of psychological state. So as long as Maria had the right kinds of psychological states (a lot of positive emotion, very little negative emotion, a belief that her life was going well), it would be right to say that she was happy.