Rabbi Israel Baal Shem Tov (1698-1760 and founder of the Chassidic movement) was asked: “Why is it that Chassidim burst into song and dance at the slightest provocation? Is this the behaviour of a healthy, sane individual?”
The Baal Shem Tov responded with a story:
Once, a musician came to town–a musician of great but unknown talent. He stood on a street corner and began to play. Those who stopped to listen could not tear themselves away, and soon a large crowd stood enthralled by the glorious music whose equal they had never heard. Before long they were moving to its rhythm, and the entire street was transformed into a dancing mass of humanity.
A deaf man walking by wondered: Has the world gone mad? Why are the townspeople jumping up and down, waving their arms and turning in circles in middle of the street?
“Chassidim,” concluded the Baal Shem Tov, “are moved by the melody that issues forth from every creature in God’s creation. If this makes them appear mad to those with less sensitive ears, should they therefore cease to dance?”
Source | http://www.chabad.org
They dance because they have tapped – in the words of George Fowler – the “unmined gold” that is inside.