There are people who fall in love every time they see someone new and attractive. Call them immature or insane if you like. Labels like that gives a clue on where to find them. Sex addicts form bonds with their partners, and positive upbeat people show it. Young singles play seduction at every opportunity, perhaps that is part of our survival instinct too. The point being, there is a vast banquet if he would have just learned to work it. Maybe he was just a couple of psychology classes and a few weeks field work away from enlightenment.

"I need blood. I need the love that's in your blood. What you're feeling about me right now. In your throat, in your chest, in your blood. Like something you can touch. Remember the crystals? Rage, resentment, malice, disappointment, despair. Emotions. Emotions are things. Your blood carries every emotion possible to feel. "

Interesting. Is this indicated in the movie, or are you speculating? Thanks.
"nothing is left of me, each time I see her" - Catullus

Because it isn't the blood per se that he needs, it's the love and emotions that are accumulated in it. He feeds off of the love, like a life-force vampire, only more of an "emotion-force" vampire.