I'm reading the book The Happiness Project. In it a young woman writes about her realization that in the middle of her good life, she wasn't as happy as she should be. So she set out to figure out how to change that. The book chronicles her research, efforts, results and realizations.
I just read that she had a realization one day that she almost had it but that something was missing. What was it that made people keep striving? And the answer (so far) was that we are happy when we continue growing.
I think there's something to that.
If you haven't read the book, I recommend it.