It is worth noting that the list usually in this site which has gone around the web quite a few times is by no means anything like a comprehensive or complete selection of graffiti from Pompeii. For example, one of the more common pieces of grafitti from Pompeii is "Arma virumque cano", the opening three words of the Aeneid (Kristina Melnor in Ancient Literacies plausibly suggested that this was because it was featured heavily in schooling) but isn't included. It isn't much of a leap to say that the particular graffiti on that list were there because they were particularly amusing.

A quick note, one of the best graffiti is "O walls, you have held up so much tedious graffiti that I am amazed that you have not already collapsed in ruin". The bit that isn't often included is that this was itself a stock graffiti phrase and shows up all over the place, which I think is some sort of irony.