Christian has low self esteem, as well as high worries that he might lose Clara. I think because of this, its when he gets paranoid about what she wears, where she looks, whether she picks up the phone the first time he calls, and just staying in contact all the time. If I were Clara, the first time Christian started to act like that, i would probably break up with him!
Also, it doesn't help that there had been a murder years back. They were a couple simply sitting at a lake, when he sliced open her throat. Clara's father has made it clear what he thinks: the boy was obsessed with the girl, the girl tried to break up with him-then BOOM. There you have yourself a dead girl. Clara's dad just doesn't want that to happen to his daughter, is all.
