It makes us think more clearly.
Helps us make better decisions.
It makes us less gullible.
Breeds attentiveness and careful thinking.
You can cope with more demanding situations.
A bad mood can promote a more concrete, accommodative and ultimately more successful communication style.
So these are my conclusions, when you’re writing your first draft be happy, jovial, and cheerful, because you need to be creative. When it comes to editing feel free to put on a grumpy face, so you can watch for details and be more attentive.
Side note for all those people who live in sunny climates - wet, dreary days sharpen memory, while bright sunny spells make people forgetful.