Elias is a thorn creature who’s never seen any other thorn creatures like himself before in his life. He lives among humans because, while most of them would write him off as terrifying without a second thought, he can count on just enough of them deciding to treat him kindly or at least tolerate him for it to be worth it (unlike fairies, who flat out don’t accept him, to the point it’s not worth it). Because Elias is committed to living in human society, because his closest relationships will always be with humans and not things like himself, humanity (and lack of it) colors just about every aspect of his life and future. Somewhere between wandering the woods and settling down near London, his usual physical form changed from a creature with paws to a creature with hands and feet (a form that’s unnatural enough for him that he’ll relax into other forms when he’s exhausted). Because humans often disapprove of his face, he went so far as to incorporate a way of covering it into his everyday wardrobe.
Elias had to develop extensive knowledge of how to act the part of a creature with human emotions, to keep his few human friends from leaving and to get by around the rest while in disguise. He not only lacks companions he can act naturally around, he doesn’t even know what it means for a thorn creature to act naturally. He never had anyone to teach him or any examples to observe.
Some of Elias’s emotions, possibly many of his emotions, do have some degree of overlap with human emotions (jealousy, loneliness, kindness, attachment, etc.), so it’s not entirely true that he ‘can’t relate’ to a human; his problem is that he’s always the one framed as being ‘in the wrong’ when there are discrepancies. He has to tiptoe around everyone he knows, and no matter how careful he is, no matter how much extra work he has to put into something as basic as socializing, they’ll always treat him like he’s seconds away from fucking up.
It’s 100% wrong of Elias to want so much control over Chise, and that’s not excusable (let her go to collage you bastard), but the itch he’s trying to scratch is being able to approach a relationship, any relationship, from a standpoint of being acceptable.