I agree with Why??, giving blood is one of the finest things you can do. You're helping someone, maybe even saving their life, before they need your help and without knowing who they are, or anything at all about them. It could be a child, or a person of a different faith (or none at all), or someone that speaks a different language. Beneath the obvious differences, the one thing they all have in common is flesh and blood.
Giving blood is a selfless act, and such a simple one. Assuming you can, you should not only give blood, you should go out of your way to do it. Ask how often you can donate and make a point of doing it, rather than waiting for a reminder letter to come through the letterbox. And as a reward you get a nice warm feeling of having done something worthwhile (even though it was quick and easy), and the knowledge that somewhere, in some operating theatre, your donated blood could literally mean the difference between life and death for a complete stranger. What a powerful gift!
If more people focussed on positive things like donating blood, things that benefit rather than divide, there'd be a whole lot less killing and bombing going on round the world. Yes, it's only a little thing, it's painless, and it's over in half-an-hour, but it's valuable and constructive. I wonder how many dimwits and morons, sitting in their miserable hiding places, attaching timing devices to explosives, actually bother to donate blood. None, I guess. Too busy spilling it. A perfect example of stupidity in action.