Somehow we become imbued with the idea that mature adults don't do that. Adults are supposed to shape sounds into words that express mature emotions. The most genuinely honest emotional expression, however, comes from the children. Observe a young child's uninhibited release of spontaneous vocal sound and enjoy an intimate glimpse into the most audible expression of that individual's Self. I contend that singing is an inborn response in those moments of absolute emotional tranquility.
Babies sing to themselves.
The fact that we recognize no identifiable melodic sequence does not mean that it is not singing. Such spontaneous oral response has sustained emission, rhythm, pitch variation, and emotional expression. Like the infant, we sing because we feel good and singing makes us feel even better. When we sing to ourselves we are, in effect, communicating with the Inner-Self. It is an intrinsic means of truly pleasing and enjoying one's Self.