I probably play World of Warcraft way too much. Admitting you have a problem is the first step to recovery, right? My addiction stems from not being happy with one character, or two, or three... I have 7 characters leveling in the Outlands right now, two of which are raiding Karazhan. Why can't I stick with just one and work on it until it's the best of the best? Because I love my alts!
I created my first alt when my primary character, a Hunter, hit level 52. I'm not entirely sure why I did so, but it probably had something to do with having out-leveled my friends by about 20 levels and wanting to help them without hindering our playing experience. My first alt was a Warlock, who I took to 60 first, becoming my first instance grinding and raiding main character. The hunter stood on standby for a while, until my guild had Molten Core on farm-mode, upon which I took him to 60 and brought him in for our alt runs. He was one of the first to complete the Lok'delar/Rhok'delar/Lamina quest in our guild, which he still has (at least, until I dust him off and bring him out of retirement).
After a few months with them, I met other people who played on a different server, and they needed main tanks for their progression. So I was off and running with a Warrior, who was level 60 in a matter of a few weeks. I helped that guild progress through Blackwing Lair when I took my first burnout hiatus. It took a few months of enjoying other things, more real-life than virtual, when I was feeling the pangs of withdrawal and logged back in.