All Write, Already!

So they say that a vampire cannot enter your home unless you invite them in.

Have you ever considered that this particular bit of the mythos was invented by someone who was really peeved about a long-term house guest?

Vampires:
They are invited in.
It's a bitch to get rid of them once they've been invited.
They drain your life force by drinking your blood.
In stories told from the perspective of a non-vampire or would-be victim, the vampires' mannerisms and dress are always ostentatious and worthy of comment.

House guests:
They are invited in.
It's a bitch to get rid of them once they've been invited.
They drain your life force by eating the last of the cold cuts that you've totally been planning on using for your lunch for work, Goddammit.
Once a guest has stayed long enough, you start to get annoyed by everything the say, do, and wear.

Yeah, I think I'm sold on this theory.