Ten reasons to give blood(Potentially relevant link)
#10 says you get free juice and cookies. True. I also got ice cream today. Birthday cake ice cream tastes like store-bought frosting.
#9 says you'll weigh less. True, but refer to #10. If I give a pint of blood and then get free juice and cookies, am I now one pint lighter? (Pints are volume, by the way. Blood is thicker than water, though not much thicker.) (Cecil adds this comment: 'Americans are so pathetic.')
#3 says blood is something money can't buy, but don't some places pay you 50USD for a plasma donation? It's also just a stupid reason. Who gives a damn about the rich and famous? You get juice and cookies, man! Juice and cookies! Get to it!