"Why are you singing all the time?"
My boyfriend: "Why are you singing all the time?" (Humming, actually).
Me: "Because I am happy."
Him: "It's the same when you are nervous."
Me: "That's because when I am nervous, I am calling my happy with this song and when I am happy I am confirming it."
That humming has kinda the same melody and it's not from an actual song. It comes out of me at different times. Sometimes it takes epic dimensions, when I am humming for more than an hour together and crying and releasing and calling back pieces of myself through different time-spaces.
It saved my life a few times.