The doffing of the hat is a sign of respect , a gesture to honor another person, and in this self-portrait, the artist honors the picture's owner, perpetually. And you the viewer, temporarily.

In the Western Christian tradition, men remove hats when honoring God or country, (note the symbol of the monk's tonsure). Jews and Muslims though, cover their heads to honor God. Jesus is never portrayed with a hat, not even in paintings where he's in the Temple, but as a Jew he must have covered his head.

The concept of honor is extremely difficult to define, but very easy to notice in the carriage and demeanor of other people. It's the quality we desire most from our friends, and from the material world as well. The body of civil law has been composed to enforce mininum honorable standards of conduct.