Also there are times when I decide I want to read my own stuff for entertainment instead of a published novel. And the joy I get from that keeps me writing.
As an aside though I feel that the science/engineering heavy HN crowd would bring some life to science fiction. We have the ability to write things that make technical sense and I'm sure many of you internally roast the handwaviness seen in some franchises. I don't know if I'm the only one that does this but I plan out the computer networks my characters use, draw cutaways and chip architecture diagrams for new devices I come up with, read scientific papers for research, and so forth. Sometimes that's even more fun than the actual writing!
The best part about writing the story yourself is that it never ends unless you want it to (or you end, I suppose).