Heiji had a way of making his enthusiasm infectious. Also it didn't hurt that Heiji was making more of a fool of himself than Kuzuryuu was. Either way it was enough to get him to lighten up a bit and try to actually enjoy the song.

They had like four minutes of this song, being miserable the whole time wasn't fun for anyone, especially not him.

And without realizing it, the longer the longer the song went on the more Kuzuryuu got into it and the louder he got. He was definitely enjoying it despite himself.