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.

[1]actually the jury is still out on that one