Haven't seen anyone talk about it but I believe happiness is partly hereditary. It's genetic (not totally, but it must play a big part, since depression is heredity[1]).
Just think about yourself and your own natural happiness level. For my siblings and I, it seems very similar to our parents. Of course not everyone is this way but I definitely believe some people are born a lot happier than others.
Isn't this the nature vs nurture debate? It seems like this could just as easily be attributed to growing up in a good/happy family. Not to say that genes have no effect, I just think the environment you are raised in has a more significant effect.
If bad genes are sufficient to make you depressed, then that doesn't imply they are also sufficient for making you any more happy in any point in time.
Just think about yourself and your own natural happiness level. For my siblings and I, it seems very similar to our parents. Of course not everyone is this way but I definitely believe some people are born a lot happier than others.
[1]actually the jury is still out on that one