Each game has it's own set of reasons why I want to make it.
Sometimes it's a feature of the language or engine like this game:
I wrote that one because I found an easy way to do physics and thought Pool with non round objects would be cool.
This one I wrote because I figured out how to simulate a tree growing and trim that tree with a database I designed:
I wrote this one because I needed a game written in only 4 hours:
This one I just wanted to implement hardware acceleration based on the last game in this list:
I wrote this one 3 times, but this time was to have a starting project on Android: