Are we supposed to have one?
I guess I'd say something similar to what Proxy said. Although, one could probably say that what he said is how most everyone operates, on a very simplistic level.
If you mean "what's your purpose?" as in what drives me and gives me a reason to keep living then I guess my motivation for almost anything is boredom. I get out of bed in the morning (afternoon, actually, I guess) because I get bored lying (laying?) there doing nothing. I jump around from activity to activity (games, reading, etc.) dropping each one as it becomes dull to me and then I'll eventually go to bed because I've exhausted the day's entertainment. I wouldn't say I really have a long term purpose -- I've got no mission or higher calling. I think at some point I'd like to live in the California Bay Area and work at a record store and/or record press but it'd hardly be the first time I had an "I'd like to do that" thought that I'll abandon months or weeks later, so I wouldn't qualify it as a purpose.