My own personal theory on emotions is that they arise out of the perceived difference between what we desire and what we result we get in reality. If we exceed our desire then we feel positive emotions. If we fail to fulfill our desire then we feel negative emotions. If you consistently tend towards feeling negative then it is because you consistently set desires and expectations for yourself that you fail to fulfill (hence the focus on what you haven't got as you feel negative emotion from that deficit).
The way to consistently feel positive and to feel good about what you have is to manage your desires very carefully so that, more often than not, you fulfill them. This revelation has worked wonders in my life. You can read a brief synopsis here:
Fundamental Emotion
Nick