Here they are:
1. Because you love other writers -- they're friendly, supportive, and just generally nice people.
2. Writing is an excuse to hide out with your computer for hours or even days at a time.
3. Saying you're a writer makes you seem more interesting than if you just say you're retired....or you watch television and crochet...or you're an accountant.
4. Being a writer gives you a better excuse to attend writers' conferences and conventions.
5. Getting published is a huge kick--huge enough to warrant a happy dance in public (although so far I've only done that in the privacy of my own home).