Why do Vampires eat humans?

VampireOne thing about vampires that really confuses me is the fact that they like to suck the blood of humans, and they get depressed and pitiable if they have to feed on some animal.

I mean, have you ever met a person you would like to eat? Okay part from the fact that it's cannibalistic and totally wrong... I'd say most people drink, smoke, or just plain eat wrong.

If I was a vampire, I'd go depressed if I only had people's blood to suck.

But then again, I know a vampire is supposed to be all symbolic and not that pragmatic that they would only hunt on a health club...