It has to do with the way Trump views women, they are only sex objects, those are the best complement he can give: I want to date you, I want to have sex with you. I don't he actually sleeps with her, he loves his daughter and he's being as nice as he can towards her. Most of Trumps comments about other women have to do with the way they look, their attractiveness, their "ratings". His daughter is no exception.
For the record, I don't think Trump having sex with his daughter is creepy. Years or decades from now, this is what we'll want to be legal. A phony paternal bond shouldn't prevent two people from doing what they want.