Most depressions indeed go away on their own, and perhaps are even helpful - they can force a person to reconsider and rearrange their life. The heavier ones probably have to be treated with everything that the patient and their environment can muster, all at the same time. And since the patient usually can't do much, it's up to the environment (friends, family, health care, mental hospitals). Please keep that in mind and force your depressed friends to go for walks, or do anything else at all, repeatedly. All the +5% basics like physical activity or smalltalk really do add up. Though the depressed person will not be able to see it for a while.