Bach, Billie Holiday, Preservation Hall Jazz Band...What is it about them?

I’m sitting here right now listening to The Preservation Hall Jazz Band and I am totally at ease. They aren’t the only group that does this to me and many other people I would assume. Any music can be relaxing in a manner of speaking. Anyone from Mozart to B.B. King can have someone in an almost catatonic state. What I want to know is what is it about music that makes a person so relaxed? Does some kind hormone release into your brain to mellow you out or what? I don’t fully understand why.