As one who's dealt with depression for 30 years, i feel i have some insight.
Depression is a bit of a broad term that does not allow for distinctions.
-There are some people who face depressive issues, and some lifestyle changes and working on their outlook can change that. I once knew a guy that seemed to have depression He had goals and dreams but felt unable to act. After giving him advice on eating better, sleeping better and exercising he disappeared for a while. When he returned he told me he took my advice and the depression had lifted and he was already working on his goals.
-Also some depression is related to physical issues. Hormones, out of balance vitamins, etc... can play a part in one feeling depressed. Usually a doctor can recommend what needs to be done to rectify the problem, and the depression will disappear as the physical problem gets corrected.
-Sometimes depression is temporary, such as after the death of a loved one. My dad went through a year long depression after his mother died, but after his time of grief, he recovered and went back to normal.
Then there is the most difficult kind of depression. Where the mind is physically wired incorrectly. 1000's and 1000's of brain scans have shown what various mental illness looks like in the brain, which parts of the brain tend to be more, or less, active than normal, depending on the mental illness.
When this happens the depression is unrelated to the persons surroundings, life, health or mindset. Positive thinking or any other such methods are ineffective, because the brain is changed, and such positive attempts are ineffective against a brain that is lacking in the right balance of chemicals.
And suggesting that such people just need to get over it, read their bibles, think more positively is kind of like telling someone with cancer to take some vitamin C and expecting that to get rid of the cancer. Depression isn't a lack of faith, either.
But sometimes pictures can make things more clear than words. Here are scans of a healthy and a depressed brain. The yellow, orange and white indicate brain activity. Blues indicate lower brain activity and black indicates a lack of activity.