I feel we love our work because we all need a worthwhile struggle and goal.
I believe some of us have an intrinsic desire to build things. Just like birds build nests and bees build hives. Some of us have a desire to build and are probably rewarded with some chemical brain fix as a result. At this point though, we've simply changed the medium from sticks and mud to Python and Minecraft.
Dopamine: http://en.wikipedia.org/wiki/Dopamine
Realizing what gives you award stimulus is important for reinforcing behavior you want and discouraging what you do not want. It is also good to know for all you app and game developers out there.