Well, sometimes It is better to be alone, than to be in bad company.
Their relationship wasn't... Healthy in any kind of way you can imagine, the main character just got crazy like her from time to time due to her "Love" everyday she kept telling to him and showing it her expression. It sure weighed his mind everyday and he couldn't take it off anymore due also to the pressure that his friends just kept pressuring him most of time to love her more, understand her feelings, don't just be a receiver, etc.
I'm sure that some way this f***ed his mind as he was slowing accepting her obsession thinking it was just something that
"normal lovers" do.
Psychological is something strong in this manga. Not just for her, but also for him.