He's crying! Tengoku really acts differently towards his students vs his brother vs Aoki. To his students, he's gentle. To his brother, he's cold and tsun (because if you can't monopolise the thing you like, you make them feel bad). To Aoki, he's childish.
Kisses hold no meaning to Tengoku because he sees it as a tool. But he seems to treasure other physical affection like hugs, headpats, and bites.
@Anidict13 Where did you read the raws?