Lighten Up

Alright, listen up, you chucklehead. The joke is that the person threw a lamp at an emo kid and told them to "lighten up". Emo kids are known for being sad and into darkness, so telling them to "lighten up" is funny because a lamp gives light. The joke is a pun, you buffoon, because "lighten up" can also mean to become happier or less serious. And you, my friend, you are very, very serious.