Familiarity breeds contempt, which is certainly true of a composer and his themes.

Scenario: A motive is written, put aside to provide some distance, and then revisited a few dyas later. Still being satisfactory, it was then choosen as the basis for a new piece. A melody is then written based on it, played a few gazillion times, sung a few more, analyzed, reworked, played some more, tweaked a bit more and then finally, a melody is born.

So by the time the melody is written I have already lived with it for a few days and heard it so many times it's a wonder I even continue with the piece at all!

From the author's over-familiarity with his subject comes the tendency to develop it too much too soon, forgetting that the listener has never heard it before.

This has always been a problem with me. That is why I often follow a few guides, one of theme being "Develop slowly, and make your development audibly linked to the main material."