Some people are simply curious since childhood and have always had and maintain this type of behavior, touch the wall with the pretense of seeing how dry it is, if it can dirty you or not.

I'm not really used to doing these things, I think it's in the psyche of everyone, since children teach you a couple of things and one that I was taught is when I was told, this hot, careful, and only checked once.

 I think it's the same as the case I mentioned, only as it causes no pain and you can clean it continue to do, but it's something that comes from childhood and everyone is different, you will find people who do this and others who do not.