lol - have to agree with knowabit!
I do think it was because of her weight loss. I read an interview by Ricky Groves who said Hannah became completely obsessed with her weight, had scales everywhere, cooked her meal separately to him and just became very distant. I think she may have been stale in her relationship, started getting attention from men, loved it, started getting more job offers, and realised she had the confidence to leave him. Oh and also the fact that she met someone else! She sounds like a total bi*ch. She may be slim but she'll never be pretty!