One of the reasons that I have trouble being happy is that I'm never satisfied. I'm always looking for something "better" and it's pretty frustrating. If you're never satisfied then you're always looking around, seeing what other people have and feeling unsettled. If you manage to acquire something, you can't enjoy it and spend most of your time with buyer's remorse.
As you may know, I'm in the process of buying a house. It's definitely a cool place, but I can't stop my brain from thinking about all the things that I don't like about it. Focusing on the negative puts me into a bad mood which causes me to focus more tightly on the negative and so on and so forth. Gah.
I'm also incredibly impatient. I just want things over with quickly. I'm not really sure why, but I can't stop to smell the roses. When I hike, I hike as fast as I can rather than hiking to enjoy the surroundings.
I will admit that this personality trait/flaw helps me be a good engineer because I'm always looking for flaws and how to improve them, but it's wrecking havoc on my personal life.