It's the fact that she has been treated badly and wronged by so many men in the past (in different ways), and he will have known how hurt she was from this. She gave him several chances to leave and he didn't take them, so she thought his love was real.
I know he was the lead in a (tiny) film before meeting her, but he'd still be 'Joe Who?' without her. She has treated him to a life of absolute luxury (that none of us can really even envisage the scale of), made him her muse, put up with his complications, got him a Grammy, introduced him to so many connections.
If I was Taylor I'd much rather be portrayed as a psycho than a fool.