What are the humorous aspects of Walter Mitty's daydreams? Why did you find them funny?

Walter Mitty is a man who is totally dominated by his
wife.  In addition, he is really incapable of doing much of anything right.  So he's
really a complete loser in real life.

That, to me, is what
makes his daydreams so humorous -- it is the contrast between how he sees himself in his
dreams and how he is in real life.  In his dreams he is brave, desirable, forceful and
respected.

What also makes this funny is how he keeps
getting jarred out of his dreams by things that make this contrast clear.  For example,
he dreams of being an important doctor but then he goes the wrong way into a parking lot
and has to have the attendant park his car because he himself can't do it
right.