If you don't, chances are you'll die, or be horribly injured if you have an accident.
"But it's uncomfortable" - Going headfirst through the windshield, and then onto the pavement, is MUCH more uncomfortable.
"Well *insert name here* was killed because he/she wore a seatbelt" - Shut the fuck up, because you're a moron. By spreading this kind of delusional crap, you might just be able to convince someone of this, and risk their life in addition to your own.
Seatbelts save lives, end of story. If you're not worried about your life, then try thinking of all the people you're going to hurt when they find pieces of your battered body, strewn along the highway.
Not wearing a seatbelt is just as selfish as suicide, because it show complete disregard for anyone but yourself.
Info and Statistics
Ok, i'm getting off of my soapbox now. I just needed to put that out there.