The Depressed Kid and the Tree

Alright, D33ZNUTZINYOJAWZ, listen up, you're not the sharpest tool in the shed, that's for sure. So, the joke is that the kid is already depressed, meaning he's feeling down, and he wants to high five a tree, which is already a sad thing to do. Then, the tree doesn't high five back and leaves him hanging, which is a double meaning because it means that the tree literally doesn't high five, but also the kid is left feeling even worse (like he's "hanging"). This makes the sad kid even sadder. It's funny because the tree is a plant and is not a person.