>>116788509Well, first off, Amity wasn't "antagonistic to everyone", she was merely a bit too prideful(as we later learn because of her parents pressure) towards Willow, and considered Luz a troublemaker. And it had nothing to do with her being human.
After Luz showed Amity that she didn't meant to cause trouble and is seriously trying to become a witch her own way, Amity realized there is more to her than just someone who annoys her.
Then the library incident happened, at which Amity realized that Luz is just a quirky weirdo, not a bully. And that she herself, by trying to please her parents, became a bully herself. They also got closer because of their shared hobby.
In the bootcamp episode they act towards each other without any prejudice and thus acted like regular friends.
Then comes the Willow mindfuck episode, and NOW Amity starts to blush, because Luz has no sense for personal space. That, on top of Luz being helpful in fixing her relationship with her childhood friend, made Amity realize that she likes Luz more than other people as she is very kind and funny.
It's a completely natural progression. A bit rushed yes, but still clear progression from A to B.
