1. People really need it--when my brother was being treated for cancer, it was amazing how much better he felt after a transfusion.
2. It's irreplaceable--donating money isn't a substitute, and no one can give extra blood to make up for what I don't donate.
3. For everyone I know who can't give because of ridiculous and sometimes discriminatory rules. All of my gay male friends, women who've dated bisexual men, people who've gotten tattoos in the past year (even with sterile needles, at licensed places), anyone who's ever lived in Africa, etc.
4. You meet friendly donors and nurses. And sometimes you get pins or t-shirts or coupons or other fun things.
5. It's a chance to lie down and relax for a while. And you HAVE to eat cookies afterwards. It's an actual rule.