We know that genetics contribute to the risk of developing depression and we also know many of the environmental and psychological exposures that further increase that risk. But we don't understand all of the neurochemical mechanisms along the way.

And from the other direction, we know some of the medications and other treatments that can help with depression, but we don't understand fully how any of them are doing the trick.

I've discussed the serotonin issue here: https://medium.com/invisible-illness/serotonin-deficits-dont-cause-depression-revelation-or-fake-news-4ce33dedd36b