Disclaimer: I Do Not Own Jekyll and Hyde.
He is unlike any man she has ever met before and her common sense tells her to keep away from him, but she's drawn to him in a way she's never been attracted to anyone before. Bella can sense that Hyde is dangerous only a fool wouldn't be able to sense it from him, however it's that danger that keeps dragging her towards him.
When he changes from Hyde back to Jekyll he no longer looks at her like she's the beautiful thing he's ever seen. He no longer wants her when he's Jekyll but she can live with that.
Because she has the moments where Hyde is free and he looks at her like he wants her, only her.