I started donating blood about 2 years ago when I turned 17 (minimum age) because my mum told me to, now I continue to do so because I feel the amount of effort it takes me to sit in a room for an hour once every 4 months is pretty much worth it to potentially save a life. Plus you get free snacks after your donation.
Another thing that excited me with blood donation is that after certain amounts of donation your donor level goes up and you get different benefits, by benefits obviously I mean your blood doning card changes colour.
From two days ago I'm on:
Which means I've donated 5 times, massive level up from the amateur, aesthetically unpleasing, 0-5 donation red card:
Disgusting...
Obviously I have a long way to go, well 29 years and 3 months to be exact before I reach...
DIAMOND
I know what you're thinking, it's purple. Still worth it. In about 30 years and 83 pints of blood's time I will have finally achieved winning status and will be able to rest happily knowing I have leveled up to the max.
For all the in between cards and "rewards" see:
http://www.blood.co.uk/giving-blood/donor-award-scheme/whole-blood-donor/