You can grow new brain cells

Researchers found neurogenesis (the creation of new neurons) occurs in adults. Certain factors can inhibit or promote new neuron growth.

Hippocampus - learning, mood, emotion, memory - new memories - new neuron growth

Exercise, proper diet, and sleep--the usual things--help mood (depression) and contribute to new neuron growth.

And, mechanistically, transcranial magnetic stimulation (TMS) (manipulated via Stanford Neuromodulation Therapy [SMT]) may help treat (extreme) depression:

Researchers treat depression by reversing brain signals traveling the wrong way - https://news.ycombinator.com/item?id=35975353 - 3 days ago (308 comments)

From the link's source,

> “It’s almost as if you’d already decided how you were going to feel, and then everything you were sensing was filtered through that,” he said. “The mood has become primary.”

But challengingly,

> “Even things that are quite joyful to a patient normally are suddenly not bringing them any pleasure.”