I am convinced that some people are just genuinely happier than others and that they aren't simply "faking it." There's always been a huge contrast between my giddy mother and my brooding father.
My base happiness level, just like my base energy level, is quite low. I'm not going to assign a number to it, but it appears to be lower than the general population. Some people just genuinely aren't good at being happy.
Armchair psychologists like to refer to this as "depression," but it's really not the case. Some people are happier than others and some people are less happy. Those who fall under the general population's happiness level can still experience happiness...it's just less often.