Pompeii is the city frozen in time. Which means that nobody ever came through and cleaned up all the (often incredibly dirty) ancient Roman graffiti (or added their own, more modern, stuff).
So, what you find is a really cool time capsule of the way random, average puellae et pueri talked, at least in certain situations. This is colloquial Latin, and that's not something we get many chances to see.
It's also hilarious. I've seen some of these examples of Pompeiian graffiti over the years, but, as far as I'm concerned, it never gets old. (Ba-DUM-ching!) Some good examples:
From the Bar/Brothel of Innulus and Papilio: "Weep, you girls. My penis has given you up. Now it penetrates men’s behinds. Goodbye, wondrous femininity!"
From the Bar of Prima: The story of Successus, Severus and Iris is played out on the walls of a bar: [Severus]: “Successus, a weaver, loves the innkeeper’s slave girl named Iris. She, however, does not love him. Still, he begs her to have pity on him. His rival wrote this. Goodbye.”. [Answer by Successus]: “Envious one, why do you get in the way. Submit to a handsomer man and one who is being treated very wrongly and good looking.” [Answer by Severus]: “I have spoken. I have written all there is to say. You love Iris, but she does not love you.”
From the House of Pascius Hermes; left of the door: "To the one defecating here. Beware of the curse. If you look down on this curse, may you have an angry Jupiter for an enemy."
From the basilica: "The man I am having dinner with is a barbarian."