I feel bad because my reason is a lot less...fun(?) than most of these. Mid TBC, military buddy tricked me into the game, and I started a priest to "help." Early WotLK, while on leave (and after a command change), I found out a few old highschool buddies played on a different server, so I joined them, but 3 of them were already arguably the top priest healers on the server, so I started leveling alts to pick from, because being shadow hurt my soul. During early Cataclysm, my at-the-time-guild's main tank quit to focus on college and I stepped up. Haven't really looked back since.