Paul Graham on how to do what you love:
You have to like what you do enough that the concept of "spare time" seems mistaken. Which is not to say you have to spend all your time working. You can only work so much before you get tired and start to screw up. Then you want to do something else—even something mindless. But you don’t regard this time as the prize and the time you spend working as the pain you endure to earn it.