From Joel on Software:
And I started to learn that the days when I was happiest were the days with lots of small successes and few small frustrations.
Years later, when I got to college, I learned about an important theory of psychology called Learned Helplessness, developed by Dr. Martin E. P. Seligman. This theory, backed up by years of research, is that a great deal of depression grows out of a feeling of helplessness: the feeling that you cannot control your environment.
The more you feel that you can control your environment, and that the things you do are actually working, the happier you are. When you find yourself frustrated, angry, and upset, it's probably because of something that happened that you could not control: even something small. The space bar on your keyboard is not working well. When you type, some of the words are stuck together. This gets frustrating, because you are pressing the space bar and nothing is happening. The key to your front door doesn't work very well. When you try to turn it, it sticks. Another tiny frustration. These things add up; these are the things that make us unhappy on a day-to-day basis. Even though they seem too petty to dwell on (I mean, there are people starving in Africa, for heaven's sake, I can't get upset about space bars), nonetheless they change our moods.
Now that I think of it, the idea that you can will yourself to happiness, something I've beleived in if not practiced all the time, isn't too far off. Essentially you are willing yourself to ignore the little frustrations.