Reasons 1 and 2 arent that important to me. The main reason I code is #3.

1) Brain exercise. I always feel sharp after a coding session, even if it ended in disaster.
2) Lots to do! There's never a full day in code. Make your own universe, if you so desire.
3) Pride. I have a pride problem. I never felt proud of myself no matter what I do. I graduated with a melancholy feeling, same deal when getting my license, same deal when passing a test (God, glad that's over!)... But code makes me proud. I love what I make. I want to show everyone. I want to show it to everyone before it's even finished because I just can't wait. I want everyone to use it and to love it. Because I sure do, and it's the best thing ever.

I could make a viral video, produce a triple platinum record, or build a billion dollar business and still not feel the same level of genuine satisfaction and happiness that I may get from writing good code.

It always keeps me coming back.