You don’t have to be happy, Carol. You just have to write with Gusto.
Does GUSTO=happiness?
Maybe.
Maybe not.
Gusto might be just getting into it, wanting to keep going, you could be bawling your face off while you do it but you still have gusto?
Maybe?
Thoughts?
Can you be excited about your writing and also be depressed?
Throwing Up Words 
I am almost always sad when I’m writing. Even the funny stuff. So yes, you can be depressed and writing. I might even be writing happily in my depressed state.