It's a habit that I find bothersome and humorous at the same time. It seems to be a compulsion. More than that, it's a way for people to prove that they do, in fact, exist.
Let me explain. The best-known works of art have been seen by millions of people around the world for centuries. The Mona Lisa might not be anything particularly special to look at, but for some reason or other, she is instantly recognizable to people the world over. Therefore, if you take a photo of yourself in front of the Mona Lisa, the fact that you exist (or once existed) becomes undeniable.
You yourself may never become an important piece of history. But the fact that you once stood near a piece of history, and that you have a picture to prove it, somehow solidifies your own realness.
It's a pointless exercise, but I think it must be somehow life-affirming for those who practice it. Or maybe not. Either way, it's an amusing spectacle for me, with my own trusty camera in hand.
And the fact that I was able to capture it means I, too, must exist... right?