@mochi
You don't need to spoiler stuff like this when commenting on the chapter.
As to why he got angry: he's a teacher. He's not allowed to have any romantic/sexual relationships with his students.
In previous chapters, he was shown to be very serious and uptight, to the point where he'd reprimand himself for simply thinking: "man, she's hot" before realizing the girl was a student of his.
So in this chapter, he needed a good reason for the indirect kiss. His reason was: "it's hot outside and I'm thirsty. If I don't drink I might get dehydrated and die."
But since he realized there's no juice left, his only excuse for getting the kiss is gone.
Yes, he could've just kissed even without the juice left, but that would mean going against everything he believed in.
He could've also just kissed before realizing there's no juice left, but unfortunately he noticed it before he could "drink".