There could be another reason behind his outwardly seeming selfish attitude. Maybe right now isn't such a good time for him. Maybe he and his partner aren't getting on too well or an illness or any number of things that could be made worse by another person staying. I'm not excusing his behaviour, I'd be gutted if this had happened to me, but maybe you could sit him down and explain how his reluctance to return the favour has made you feel? And if he hasn't got a good reason, then I'm afraid that you have to put it down to experience. But I do feel for you. We like to think people will treat us as we treat them, but it doesn't always work that way.