Who knows. I can guaruntee I have a crummy gene for this. I spend a great deal of effort being not depressed. Let alone happy.
Try exercise (running), its the only thing that keeps me sane.
You are not alone.
You may find my #46 interesting.
It was surprising to me that a tiny, tiny, TINY bit of the St. John's wort herb -- much MUCH less than in a capsule -- mixed in water, taken only occasionally, can affect mood considerably.
I believe we as a group are using our brains far more than our distant ancestors. As a result, our brains are often suffering from something akin to a sports injury due to overuse.