There are physical and emotional causes of depression. Physical causes can be prescribed drugs, unintended drug combinations, recreational drugs, quitting coffee, quitting smoking and chronic aches and pains, to list just a few causes. Emotional causes mostly have to do with turning the feeling of anger inward and beating yourself up by telling yourself you're no good or that life is no good.
Sometimes a gland such as your thyroid may not be functioning properly, and when estrogens are out of range in both men and women, depression and anxiety follow. Lab tests such as TSH (thyroid), estradiol, progesterone, DHEA and testosterone can be easily done to find out if there is a problem.
Some supplements like vitamin D and DL-Phenylalanine help prevent mood change, especially those connected to not enough sunlight. Several supplements are exceptional in lifting depression. The newest one I've found is Uridine, which can lift mood and in many improves mood. The amino acid L-5HTP, when given in the proper amount, is as effective for many as Prozac, but without the nasty side effects (just don't use it along with it or any of its prescribed cousins).
SAMe and Saint John's Wort are both useful, though SAMe shouldn't be used if the depression is bipolar, and Saint John's Wort can affect the dosage of some prescribed drugs. Studies have shown fish oil to be effective in reducing the degree of depression over time. L-theanine is widely found in wide use in Japan. Benefits include; promoting relaxation without drowsiness, improved learning and concentration and reducing the stress and anxiety that may set off depression.
There are many ways to deal with depression, no one needs to suffer endlessly, and life is too short not to feel a range of emotions including happiness.