One quart low...

Gave blood today. Had a friend whose father was a hemophiliac and died of AIDS from a transfusion, and she got me started -- I hate needles. Then I got into a car accident and received blood products, so they wouldn't let me give. This scared the shit out of me, so I quit for a long time. I almost always get really sick or nearly pass out. I tell myself I'm never going to do it again, and then I do it.

It saves lives
Other people need it more than me
Humans are designed to bloodlet periodically. It's good for the system.
I quite probably will need some someday -- might as well be my own.
I've needed it several times before.
Like most things in my life, it scores brownie points
It makes it really easy to get caffeinated/drunk
Like a lot of things in my life, it's a habit that started with a girl

Everyone should give blood for at least some of those reasons right there.