He loved her enough to know she was better off without him

You may choose your friends, but not your unlikely saviors,” Magnus said cheerfully.

He speaks of you only with the greatest pride, Will

Now very much against her will, she thought of the way Jace had looked at her then, the blaze of faith in his eyes, his belief in her. He had always thought she was strong. He had showed it in everything he did, in every look and every touch. Simon had faith in her too, yet when he'd held her, it had been as if she were something fragile, something made of delicate glass. But Jace had held her with all the strength he had, never wondering if she could take it--he'd known she was as strong has he was.

My heart tells me this is the best and greatest feeling I have ever had. But my mind knows the difference between wanting what you can’t have and wanting what you shouldn’t want. And I shouldn’t want you.

She hated that will had this effect on her. Hated it. She knew better. She knew what he thought of her. That she was worth nothing. And still a look from him could make her tremble with mingled hatred and longing. It was like poison in her blood, to which Jem was the only antidote.

She did not belong to Will-she was too much herself to belong to anyone, even Jem-but she belonged with them, and silently he cursed the Consul for not seeing it.