Also the fact that dragons have a much longer lifespan that what humans do, which means even if they end up together, he'll have to watch her grow old and die in front of him and not be able to do anything, with the chance of him going on rampage that no one would be able to stop, even if it is to either bring him back to himself or to put him to rest (there's a chance of another Caller being born if a 100 years plus the years it takes for them to grow up, it doesn't sound like that happens very often, and I don't think he would even be able to fully accept them into his heart as had with Sena).
It's just.... heartbreaking when you think about it, you know?
See, this is why you don't choose the humans over the dragon, gurl. Its ALWAYS the wrong choice.