Probably has something to do with the American work morality--the zealousness we apply any religion can only weep in envy of. We believe/have been brainwashed into believing work is what we were born to do. As to how much we should do; I'm not sure this is a question for psychological studies so much as a question of how much (and of what kind of) work we actually want to do. It's like asking how many hours one should spend cleaning their house; one balances a cleanliness level one can live with against time one would rather spend doing something else.