Humans like to brag about having thumbs, which is certainly an advantage in a natural world in which thumbs are rare. The idea of animals stealing or appropriating this advantage is humorously disturbing, especially when it comes to the animals that live with people, like cats. A lot of cats always seem to be plotting to steal things from their human owners anyway, and they always seem to be watching and waiting. Imagining a conspiracy among them is easy when people let their imaginations run wild.
This milk commercial plays these related scenarios for laughs, thus allowing people to associate their products with some of these memorable images. It makes for a silly commercial that manages to stand out among most milk commercials. Many people deliberately seek out funny commercials on the Internet today. People all over Facebook would probably respond to this funny commercial in particular, especially since it doubles as a cat video.