In different countries, there are various reasons, some better than others. Chronic illnesses like hepatitis, having had gay sex, having lived in Britain in 1980's. These contribute some risk to donating blood.

Also having malaria, paying money or sex for drugs, lived in Africa for 6 months, ever had a blood transfusion, dura matter transplant... the list goes on. In Canada, every time you donate you answer about 13 questions on your own, then a nurse asks you about 10 "high risk" questions. You can tell they could recite them in their sleep, and I'm almost at the point where I could as well.

Men who have sex with men; intravenous drug use; are the two big restrictions that are common.

http://www.blood.co.uk/giving-blood/who-cant-give-blood/

There are lots of arbitrary rules out there to minimize risk. I lived in Sweden for more than 6 months before 2004 so they deem me a BSE risk. Oddly enough Sweden didn't have any cases of BSE until 2006 so I'm not sure about the logic behind that one.

it doesn't really matter, does it? I wouldn't ask here, it's not everyone's business.

(not that it implies anything specific, it's just not really strangers' business - OP has detailed profile info up. I would ask in private if you really want to know.)

I think he implied that it is for a dumb reason why he isn't allowed, that's why i asked. But yeah, i shouldn't.

If the commenter hadn't intended for someone to ask the question why did they mention it? Conversational protocol dictates the hearer engage with what was said. Asking for more information on one or more aspects of an open communication is normal.

One major reason is if you were in the UK for more than 6 months between 1980-1996. You can't donate blood in Australia afaik if this is the case.

This was due to the BSE, 'mad cow disease'