Writers need to be present. They need to notice things and take note of them. They need to be aware not only of their own feelings but the feelings of others, and the various ways of expressing those feelings.
I find it nigh impossible to do this when I'm depressed.
Depression and sadness pull you inside yourself. You're focusing on your own stress, your own problems. You don't as much notice the sadness in the voice of the boy at the register or the sun breaking through the clouds because you are worried about how much longer your grandmother has to live, or how you're going to pay rent next month or a wide variety of other things. Sometimes we have big things we're dealing with, or just really hard things, and we can't help but be internal and sad for a while. That is fine.
However. It does not a growing writer make. When we are not stuck inside ourselves we notice things about the world. When we pay attention to how other people are feeling it gives us a fresh set of eyes. And I really, really need to do better at it.
The cool thing about this is that it works the other way around too. When you're down and low, make a conscious decision to notice something around you that you haven't noticed before. Take a closer look at a tree, at someones face. Pay attention to another persons emotional cues. Before you know it, hope and even a little happiness have come back.
Happy and in a good emotional place for writing. I think we all want that.