He gained an understanding of one emotion more, so it makes sense his singing would be affected. For the better, in this case.
@no168_92 I've no recollection of if it has been stated. She seems pretty young, though, so I'd guess below 25. If it's so, the only things absurd would be the student-teacher relationship, plus the minor-adult one, naturally. Both of which are taboo. However, once he's 18, she still won't be an old woman, which is the important part. All in all, if he didn't have her, he would probably grow into a much worse man, so even if their relationship remains ambiguous, it wouldn't necessary matter.