Things weren't great with Buffy and Drew for a long time... if ever.
He had a thing for her while he was alive and after he decided to leave John and George's apartment
he began haunting Buffy.
We'll find out she put up with it because she did like the attention he paid her, for a while. Plus, being a ghost he didn't have stuff all over the apartment-- besides the pots like from the movie "Ghost" that he thought was a romantic gesture, but we know Buffy didn't like them
And when Drew gained a body full time, things were just too much for Buffy and she kicked him out... since it's a lot easier to do to a guy than it is a ghost.