Back to the topic!
It was established way back in The Making of Star Trek
that GR tried to keep things light as possible within the constraints of the pressure making the show. The small practical jokes like putting a fully made up Ted Cassidy -as - Ruk in Gene's office to meet an unsuspecting salesman, the small after work parties (which was a Desilu tradition anyways), the memos with the sarcastic edge to them. It's all part of relieving the pressure a little.
Favorite Justman memo of all:
TO: All Concerned
FROM: R Justman
I plan on being extremely mean today.
There's a reason the Trek bloopers are some of the best of all time....