This is actually quite interesting philosophically. Considering she can blend in among humans, she must have a degree of human psychology understanding, otherwise she couldn't act like a human at all. She even understands emotions to some extent. Yet naturally she couldn't ever be sympathetic because humans are her source of food. The fact she's living with and had sex with the dude, adds another layer to it. That seems quite pointless for her survival as a species, unless she indeed can breed with humans. Bjut then again, if her monster species actually also enjoys social interactions and sex, then, considering she can assume a human shape, she might be using the dude as a substitute for one of her own kind. Actually falling in love with a human seems like too much of a stretch considering she eats humans.