"Anhedonia." There's a new word.

I completely understand what you seem to be saying. Ecstatic, exuberant joy isn't something I feel often, and I prefer to keep my excitement a bit understated. I prefer the calm, content sort of happiness over the extroverted enthusiasm one sees portrayed in film and TV.

It's probably a combination of nurture and nature. As a Minnesotan, this highly educational PBS feature explains a lot about the culture where I grew up.