Seems like an odd question, but I’m sincere.
Just occured to me in the car the other day as my SO and I were singing along with a song we were barely familiar with.
Why do humans feel the urge to sing along when they hear music?
We can, obviously, listen and enjoy music without participating, but we seem inclined to want to sing/hum/whistle along as well.
Why?
'Cause it’s fun. I’m not saying this to make a joke. People do all manner of things for no more reason than the simple joy it brings. Singing, either by yourself or along with a song on the radio is one of those things.
I never sing alone.
I always have a full band behind me backing me up. Noone else can see them, but they’re there.
I don’t, because I can’t stand my own singing. I would rather do just about anything except sing. The idea that they sing a lot of songs in Heaven, has me thinking that maybe the alternative isn’t that bad. This is not putting down singing, just that in my life it is a round peg in a square hole.
Oh, how true. I sing along with the radio as loudly as I can. I play it very loud in an attempt to drown out my horrible voice. It just makes me feel better.
What frightens me is how I can remember the words (and tunes) to songs I haven’t heard for YEARS, didn’t like at the time and still think are purile - yet when they come on the radio, I up and join in like Pavlov’s Dog Chorus.
I just begin to wonder what other things I have tucked away in the recesses of my cranium, just waiting for the right signal to come along and trigger their release…
Gp
Because music gets in your soul. It doesn’t just hit your ears so you can enjoy it and then bounce out, it actually digs under your skin and gets inside you.
I’m a composer, can’t ya tell? 
You wanna know what the worst is? When your singing along to a song that is sung in a different language, like that one that was popular in the early eighties, sung by that blonde chick.
Non-English speaking people do it a lot more often than we do because of the way our record companies pump our songs over their airwaves. A Spanish exchange sudent my family hosted when I was in high school would always sing his rendition of “When a Man Loves A Woman” that would have everyone within earshot buckled over crying their eyes out in laughter. Much better than the original, IMHO.
No solid answers here, despite the fact that I’m one of the most compulsive sing-alongers I know. I’ve always felt that it has to do the tendency of humans to be highly social creatures. We like to do things that make us feel as if we’re part of a group, and singing along with songs is a very visceral way of doing that. It doesn’t matter if the music is recorded–our instincts don’t make the distinction very well. This is one of the reasons that I frequent Ren Faires and one of the local pubs that has live Celtic/Ren/shanty music–audience participation is generally encouraged, so you get entire crowds of people singing along. It can be more moving (or much sillier 
) than anything I’ve ever just sat and listened to.
You can’t harmonize alone.