A is an alien, born and raised on some other planet, and has always been obsessed with humans. They finally got the chance to go to earth, and with looking pretty much exactly like a human, it would be easy to blend into society- if they weren't so weird. Once on earth they needed a place to stay, so instead of going somewhere secluded, they decided to walk right up to the door of a random house, which just so happened to be B's house.