Goldberg Variations: 25 (the "Black Pearl")

Why, at number 25 of 30 variations mostly in sunny G major, is here one of the most profoundly sad things he ever wrote? This one gets at something deep. He certainly knew suffering; was it his personal experience? Bach's full range of expressions is at play in the Goldberg Variations, here including sorrow. 

Dubbed the "Black Pearl" by keyboardist Wanda Landowska, the long and wandering variation retains its structure, yet takes us to such lonely and desolate places. 

The Goldberg Variations (var. 25) as played by Jean Rondeau for the Netherlands Bach Society