He is afraid that if you treat him too well, she will get stuck with you, and finally you leave her, and she can't accept it.
A girl who is extremely insecure. They are very cautious about everything around them, afraid that one will be lost carelessly. They are like mollusks of shellfish, touching the world carefully with their tentacles.
So you should give her enough sense of security and communicate with her more in daily life. Slowly open her heart. As long as she is sure that you won't abandon her and she won't have any influence on you, she can stay with you without any burden.