Emo High Five

Alright, listen up, you absolute melon. This "joke" is playing on the emo stereotype that they are sad and isolated. The tree leaves the emo kid "hanging", which is like, a metaphor for rejection, but also a literal description of what happens when someone doesn't complete a high five. So, basically, the joke is that the tree, like, rejected the emo kid. And you probably get rejected a lot too because you are annoying.