Sometimes because they are bad at singing, so they need to be drunk to enjoy the sound of their own singing (compare listening to badly performed music).
I used to believe I was very bad at singing (mainly because my parents told me I was), but I sang karaoke (when drunk) a couple of times and noticed that people actually enjoyed me singing, so I started singing karaoke regularly, and I still do so. I've had a few people contacting me on Facebook and wanting to get to know me because they think I'm such an awesome singer -- if three years ago someone had told me this would happen, I would never have believed them.
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?