Being in a good mood can be a catalyst for trouble.
When you are feeling happy, you let your guard down. And the workplace isn't the place for that.
A good mood can lead to attempts at humor that offend certain co-workers.
A good mood can lead to attempts at fun flirtatiousness that veer off into harassment territory.
A good mood can lead to attempts at rapprochement with awful co-workers who will proceed to remind you just why you avoid them.
And on and on.
So be careful about good moods. They can lead to all sorts of unhappy outcomes.