This person didn’t want to confront the problem head on and say these people can’t clean after themselves, so he found a funny and creative way to do so – putting it on the grocery list. But much like patience, and unlike milk, I don’t think this is something you can get at the store.
Adding the milk to the list, though it seems as if it was added by someone else, helps drive the point home. It emphasizes the passive-aggressive tone of the request and makes it more casual, and if you ask us – more convincing. Obviously, adults know how to clean up after themselves, and if they don’t – it’s a choice. Hopefully this note will touch at least one person in the office.