I've always been a bit of an alt-holic in MMOs, namely because most out there only allow you to play one class per character, so if you wanted to play every class, you had to make more characters.

Though, even with XIV's "all jobs/crafters/gatherers on one character" bit, I always end up with new ideas for either character looks or names and not wanting to buy a name change/fantasia for it. Or I just mostly want to try out maining a job on one character before really working on it on my main (namely the tanks and healers).

I also make alts to avoid people. If I'm in the mood one day to not be bothered by friends and/or FCmates, then I have a character I can fall to until I'm out of that particular mood.

And the last reason I make alts: to re-experience the story, usually as a different job than I originally did it as.