I am more happy if I don't treat work as work

I tried two approaches to working - 1) treating work as work i.e working 9-5 - just doing what's asked of you 2) working more hours if necessary, being proactive, working hard for the project/promotion.

In both the approaches, you get the same amount of money - even though you might get promoted slightly faster with the second approach. However, I experienced that I am more happy and satisfied by following the latter approach.

Why is it so? My experience is contrary to the general wisdom that you should treat work as work and find happiness in other things.