In this day and age, many people are affected by depression. Thus, in 2015, nearly 5 percent of the world's population were affected by this. Due to the longer life expectancy, this trend is even increasing. Especially in old age, this is difficult to heal, as the affected person often can not do sports anymore and are physically restricted.
Possible reasons are often drugs that trigger depression, poor diet, isolation from other people, etc.
Professional help is probably the only correct way.