This weird behavior comes from the way cats communicate with each other. In the cat world, presenting the butt with the tail turned upward means, “No kidding, I’m here, you can see me, take a whiff.” The scent is way more important for communication in the animal world than it is for us.
A cat will scent-mark his territory, especially where it overlaps with other cats’ territories, and from these marks, other cats can sniff out useful information about the author. If two cats meet, this mode of communication gives way to visuals. At the same time smells are a great way for cats to learn about each other.
Sniffing noses is the first step. A cat will examine scents given off by glands in his counterpart’s forehead and cheeks. This he can do without turning his back. As you may know, turning your back on another party in the natural world is risky if you don’t trust that party.
On the other hand, when a cat shows trust, he raises his tail, which is in itself a friendly gesture according to cat’s tail language, then lets the other cat get information through the perianal scent glands—i.e., presents his butt for a good sniff—and if he’s lucky, the other cat will allow the same. Communication and greeting is why cats stick their butt in your face.
This behavior could be compared to a hug in the human world (though it’s best not to compare human and cat body languages as they are, to say the least, rather different). But then again, we rarely hug random persons, right? Just the ones we trust. (It is true, though, that hugs give us information through scent. If you like someone but aren’t sure how, chances are that you unconsciously like his or her smell.)
Have you ever been hugged by person you’re not that familiar with? It usually feels uncomfortable, at least on some level. You can try to play it cool, but you simultaneously hope it never happens again—and cats can experience similar feelings.
Presenting his butt to another animal places the cat in a vulnerable posture where he can’t effectively defend himself. It is a behavior reserved for parties the animal feels comfortable with; thus, if your cat shoves his butt right in your face or sits with his back turned against you, you should feel special. You and your cat have a great bond.
