For those who confused, he's jealous. In china, being jealous synonyms for being 'sour'. so when the black hair guy bought him lemonade (sour), he feels like he is being mocked by him which is why he hits him haha.
how can tachibana say no to tendou about her own damn feeling?? argh this manga is so complicated but also addictive. thank you so much to amplified for translating ??