Everyone is biochemically unique, so some will do better as vegetarians, while others will need meat. On the other hand, carbohydrates boost serotonin production. You may be subconsiously self-medicating every time you lurch for a piece of chocolate cake. I like Dr. Judith Wurtman's book, The Serotonin Solution, with its different recommendations for boosting serotonin production by separating small protein meals followed by small carbohydrate meals. Fiber is important to keep the digestive system working, and because there is such a high correlation between digestive and mental disturbances, anything you can do to clear, then assimilate and eliminate more easily will also help you mentally.
Water is essential for running the central nervous system.
Your entire nervous system, being electrical, runs better when you are adequately hydrated. People who are chronically dehydrated tend to be tired and depressed.