Donating blood doesn't affect me physically negatively in any way, and not everyone is that lucky, so I think it is important that I try to donate blood pretty regularly.
The first time I was old enough to donate blood and they had a drive at my high school, I donated and was happy about it.
The next year, I tried again and was turned away due to tachycardia (which we later found out was caused by some medication I was taking). It was annoying, because they were like, "just calm down!" and I wanted to say, "I'm frickin' zen over here, okay?" lol.
Since then, I've only been able to donate once. I've tried time after time, but each time I'm either anemic, have tachycardia, or have had a piercing too recently. I also couldn't for a year because I had a small bone graft in my jaw.
I always feel bad when they're trying to recruit people to donate blood and I can't for some reason or another. :(