Will knew that you were straying away from him for a reason, one that he didn't know. He was relentless in his pursuit to have you. It took that one look during the summer of his 14th year to make him realize how badly he wanted you. He was going to take that chance no matter what it cost.
He didn't know why you didn't give in to him already because he knew you had harbored some of the same feelings. He didn't understand. But he never stopped. Because of his persistence, he was slowly breaking down your walls.
After vandalizing Edward McClanahan's grave and sleeping together, the next morning, Will knew that everything had changed. Despite wanting to continue whatever they had, he could feel you pulling away.
"It's fine. It's like nothing happened." Will said, getting dressed. He couldn't handle being rejected again by you, so he pulled away in return.