I think it was that she was a prime example of the sort of shift in world view that he was experiencing (AND that ultimately allowed him to regain his power). She's passionately dedicated to her work even when faced with having everything she cares about (her research) taken from her. He knows that passion and what it's like having it ripped away from you right when you were on the verge of something great. That's the same situation that he's facing when he attempts to get her notebook and Mjolnir back when he raids the camp.
When that fails, he's still used to thinking of humans as just 'mere mortals' but he's left with nothing, and that makes him look to the only thing that he's got which is them. Not only id they not treat him like the crazy hobo that he almost certainly was, they're still helping him and being hospitable to him when he's lost everything else, and that means a lot.
He decides that if that's his fate, he's going to face it for them, even if it means dying as a mortal. It's the experiences that they share that ultimately allows him to regain his power, which is a big character and emotional development for both of them.
(Plus, some romantic things just click but that believability is pretty much contained in whether or not you're buying their chemistry, which apparently some folks do and others don't).
X