Seems like a good tie-in to the 30 Days of Writing Questions and the Who Do I Write Like Meme.
Bold any reasons that apply to you, strike out any that don't (if you feel like it), and add three reasons of your own to the bottom.
1. To explore themes that I don't get to see in mass media using characters I love.
2. Because it's fun.
3. Because mass media does a crappy job of representing my race* and sexual orientation. *In my Caucasian case, for 'my race' substitute diversity.
4. Because I can get more people aboard my ship writing a story than a manifesto.
5. Because TV science-fiction doesn't explore its science-fiction premises in enough depth.
6. Because it's a gift I can give a stranger and know they will enjoy it.
7. Because I resonate emotionally with the characters what I read and watch, and want to find out why by writing about it.
8. Because every tale is a universe, often with fascinating nooks and crannies that the original author never explored.
9. Because I've made some of my best and dearest friends through this very wacky hobby.
10. Because the world the creator made is vast, and I want to see more of it.
11. Because writing as a communal experience is amazing.
12. Because I can.
13. Because I get so much enjoyment from reading fan fiction that I want to contribute to the community by providing enjoyment for other fans.
14. When I become interested in particular actors and/or characters, sometimes I want to explore them in different contexts than their canons would ever allow.
15. I enjoy writing but I feel ambivalent about trying to get published; writing fan fiction frees me by removing the pressure to sell from consideration.
16. Because of 'tribbers copies (I didn't have a lot of money so I contributed drawings and stories to zines I wanted free copies of.)
17. Because I have a talent for writing humor, even for not-very-humorous shows, and I love it when people laugh.
18. Because I was once part of a fun fannish clique of talented women who encouraged me to write, talked story with me, and applauded what I wrote.
19. Because my ship will never be canon, no matter how much I might like it to be, and therefore if I want to see it I'll just have to write it myself.
20. Because the writers are not going to devote a chapter or an episode or an entire movie scene to my favorite character's hobbies and pastimes, and I like writing that sort of thing. 21. Because it's writing which is fun and relaxing for me to do in the free time when I'm not working on my original fiction and/or trying to get published.
22. Because the feedback I get from folks in fandom is so amazing and makes me feel like even if I'm not a Big Published Author, what I write has the capacity to touch people and make them laugh or cry.
23. Because good fiction always leaves unanswered questions, and this is just one way of answering them and finding resolution.
24. Because I can write and plot, but I'm not good at creating my own characters, and it's so much easier to find things to do with ready-made ones.
25. Because I enjoy seeing how character react to situations their canons will not present them.
26. Because sometimes making something darker and grimmer can be fun (see cartoons like Biker Mice from Mars)
27. Because it's helped me improve my writing.
28. To subvert canon so that the events shown on screen make sense with my interpretation of the characters.
29. For the feedback, good or bad.
30. Moar smut!