He was an autodidact, starting out as a bricklayer and being so in love with music that he became member of an orchestra, then of the Sing Akademie, then the leader of the Sing Akademie. He taught both Felix and Fanny Mendelssohn as children and youngsters (and is responsible for Felix playing for Goethe in Weimar and getting compared to child!Mozart, whom Goethe saw as a teenager in Frankfurt playing together with Nannerl). (Err, Goethe was the teenager, when seeing child Mozart. Mozart was eight or seven, I think.) There's a lovely Goethe characterisation of him: „In Gesprächen ist Zelter genial und trifft immer den Nagel auf den Kopf […] Er kann bei der ersten Begegnung etwas sehr derb, ja mitunter sogar etwas roh erscheinen. Allein, das ist nur äußerlich. Ich kenne kaum jemanden, der zugleich so zart wäre wie Zelter.

("In conversation, Zelter is a genius and always hits the ball in the corner. He may appear a bit rough, sometimes even rude when you first meet him. But this is only the outside. I hardly know anyone who at the same time is so tender as Zelter.")

In addition to coming across as sympathetic, I think "Du" was partly because he was a self made man who rose from humble circumstances and who combined passion for art with at times gruff manners. I don't think Goethe (who himself rose from middle class commoner to ennobled goverment official courtesy of Carl August) would have offered the "Du" to a nobleman. (Especially not as an older man.) But to former bricklayer (that's why he renovated Nicolai's house, btw) Zelter? Absolutely.

And that's a gorgeous sonata indeed. Which we now know Frederdorf loved!