Actually, Weasleys, it's not rubbish.
I have very narrow, difficult to locate veins. Intravenous injections are acutely painful for me. In all other respects I would say that I have a high threshold of pain, (I took no pain relief when giving birth), but I have a problem with this.
As yet (and fingers crossed), I have never needed a blood transfusion, so I've no pressing need to feel guilty about it
Apart from that, if anyone had my blood, they would immediately and irrevocably pissed.