In my approach to this piece, and in my teaching, I've always said that this is generally not "happy" music. And that has always made sense to me, considering that Hindemith was German, and this piece was written in 1939, just before WWII broke out. The Nazis were in power, etc.
Well, I found out that Hindemith's wife was Jewish. This was news to me. In 1938, Hitler's anti-Semitic campaign was already well under way, and Hindemith and his wife emigrated to Switzerland. Then in January of 1939 - the year the Horn Sonata was written - Hitler officially announced his plans to exterminate all European Jews.
In light of these facts, it really is no wonder that the Sonata has always seemed a bit dark and aggressive to me. There were some interesting things I came cross about Hindemith's relationship to the Nazi party, too. It seems he even wrote some music for them - I can only imagine that it was done under duress or in fear. But that's all beyond the scope of this blog for now.