"The blood is the life" is both a biblical tenet, and a fact. Without blood rushing through our veins pumping oxygen into every cell of our body, our bodies stop. Full stop. Die. Bite the big one. We need pretty much every drop of blood we have safe inside our veins. Our blood is our life. So it behooves us to keep our blood firmly inside our own veins, right?
Then donors go out and share it, donating it willy-nilly to blood-banks and vampires. Why do they do it? Well, in the case of donating to blood-banks it is quite normal and everyday. A kind effort at sharing from one human to another, and honestly, we can share some without ill effect. But donating life-blood to vampires, now that is the proverbial horse of a different color. Technicolor even. Why do it?
All the usual reasons, one suspects. A search for love and glory? A hope that quid pro quo will be in effect and one will eventually go from donor to donor-ee? The hope that sharing a bit of the good-stuff will satisfy the vampire and the donor gets to stay alive? The purely Samaritan impulse to be of help to someone in need -- even if they are no longer quite, well, living? But this is a kind impulse nonetheless. A combination of the reasons? The reasons will be different for each donor, as will the requests whispered by the vampire. The vampire will say, "save me."
It is human to attempt to save someone, anyone, who needs saving. Loosing that urge, arguably, means that one is becoming a bit less human. I believe strongly in free will, in personal choice, and the reasons to become a walking McMeal are personal. But remember, the blood is the life.