It's hard to figure out Kurt's expression and what he's thinking. At first I thought he was upset because he knew something was wrong. On subsequent views, he doesn't seem so much upset as surprised with the way Blaine is singing the song like he had no idea something is bothering Blaine. Blaine, however, is on the verge of tears. Did he start out that way or was the song too much for him as he contemplates losing Kurt? It almost seems like Kurt is confused as to where that came from and not sure what to do now. That kind of plays into the little snippet we saw in Thursday's promo but who knows how all this will come out in the final edit.