I recently listened to a podcast where the guest talked about the medical benefits one gets from donating blood. Men can get iron heavy blood, which is an oxidative stress. By donating blood on a regular basis, you force the body to create new blood, which won’t have the metallic load.
From the article The Incredible Health Benefits of Donating Blood by Kristie Leong M.D.:
Studies have shown that, in general, Americans tend to consume more iron on a daily basis than is necessary for good health. Ingestion of quantities of iron beyond a certain quantity can promote formation of free radicals in the body. Free radicals have justly earned their reputation for causing cellular changes which can disrupt normal cell function and increase the risk of certain chronic diseases such as heart disease and cancer. This is more likely to be a problem for men and post menopausal women since women of child bearing age shed excess iron through regular menstrual cycles. If a person happens to be a red meat eater, the risk for iron overload can be even higher.
After learning about this, I raced out to donate blood. However, for the first time ever my blood was rejected. Turns out that my visit to Cambodia last winter means my blood is at risk for malaria for up to 12 months. Come December, I intend to get back in the habit of donating blood on a regular basis. Either that or I’ll take a trip to another country with a high risk of malaria. 
Interesting stuff. I read some articles on the health/therapeutic benefits of giving blood several months ago. Since we have a blood mobile come to my workplace every other month it was easy to sign up and give blood the last two times it was here. And while there is a certain ‘good feeling’ for having helped someone else I’m not kidding myself…I did it for selfish reasons primarily. I’m not ashamed of that.
Regardless of what the techs and everyone else will tell you…I felt a bit tired for a couple of days following each donation. Serious exercise was out of the question. However, I’ve felt healthy and strong pretty much the rest of the time so perhaps there is some benefit there.
[...] Yesterday, I collected my 1 Gallon Donor pin after donating my 8th pint of premium grade B Positive. Now some lucky chap will soon have pig uterus coursing through his veins. Back in my Florida days, I donated a full gallon to that state. I like being able to help my fellow neighbors by donating blood. Either that or I’m doing it for selfish reasons. [...]