I think, if you court and marry Sebastian, the implication is that he falls in love with the farmer. (I'm not romantic so I don't personally get it, but I understand it's a thing people do.) And, along the way, once he moves in with you, he realizes that he just wanted to get out of that basement room and to get some space from his family. He still has his motorcycle and can go on rides to get away, but now he enjoys coming home to you, to a home he makes with you where he finally feels like he belongs. At least, that's the story that makes sense to me.
Plus, he loves frogs too much to be happy living in Zuzu City, where he can't watch them every rainy day.