Why do vampires ask if they can come in your house?

This is not a trait common to all vampires but one of a particular vampire which, once noted and included in literature, is now, erroneously, applied to all vampires. Vampires are very much the same person they were before they died and are now unable (to a very large degree) to change their personal habits. So if a person was polite enough to wait until he was invited into a person house while he was alive he will retain that trait once he is a vampire. I do not know the particular instance or individual that lead to this supposed trait but I know of quite a few where it was violated.

Count Dracula waited for Renfeld to invite him in to the sanitarium however he had no problem Breaking-in to the home of Lucy Westenra a total of three times.