I read in the New Scientist one theory about why we love to sing (and dance). The theory goes somewhat like:

It is evolution at work. For a tribal, pair-bonding species to co-exist as that we had to do communal things, and bonding is essential to avoid serious conflict. Singing and dancing serves little functional purpose unless you consider the "tribe's" survival. The fact is - it has, even today - see Dunbar Number. We have evolved into a singing, dancing species, because the "tribe" offered evolutionary advantages, and look how popular singing and dancing is still - particularly in the young. Soccer matches (anywhere in the world) will illustrate how we haven't shaken off the tribal tendency at all, either! And communal singing on the terraces is de riguer (in the UK at least).

A point from the lecture - exhale lowering the heartbeat. Consider the sigh when danger passes or the realisation of a beneficial resolution to a story.

Thanx 4 the lecture. As an engineer I love to hear about mechanisms and nuts and bolts. I sing (and dance) because I love to, but if I want to improve - those nuts and bolts had better fit.
I still feel the tribal evolution came first, and the nuts and bolts really support that. I guess as a species we social groom aurally more than physically. Hence folk clubs/choirs and chorus songs.