Depession (Not a poem)

What is depression? I mean, I know what it is, but why is it? Is it there to teach us? Warn us? Help us? It seems illogical to believe that it is there for the soul purpose of harming us. Most feelings are there for a reason, and never to harm us. Sadness is there either as a form of empathy or perhaps for mourning. Anger is a way of showing someone that something has upset you and they should most likely leave you alone. Stress is a warning of danger. So if all these feelings have a purpose, then why is there depression?
Perhaps that if it wasn't for laws, human fears, and modern medicine then it would be just another form of natural selection. Current laws have made it illegal to kill yourself or to help with suicides. This makes it harder for those who are depressed enough to take their own life. Human fears have now caused people to not just think of death as another natural part of life, but to begin to fear it. It is unknown to us what happens after life, so we fear it. This, as the earlier statement, also makes it a challenge to kill yourself. Even those with great depression do fear what will happen when death claims them. Modern medicine is the most powerful though. This medicine, which for all we know may only be placebos, helps get rid of depression. It takes the thoughts of suicide out of one's head and helps you destroy bad feelings. It causes you to float in an ocean of indifference. Once the person loses their sense of caring then they can no longer bring up enough concern about them self to commit suicide.
If we were to get rid of all the laws, fears, and medicine, would depression simply be natural selection? A way to get rid of those of us weak enough to be overcome by these feelings and kill themselves? Imagine an earth where every human that has attempted suicide died. Imagine again that not only did those people die, but a large percentage of those who have though about suicide also managed to kill themselves. The population would be considerably lower and there would be next to none with depression. Perhaps depression is not simply a chemical imbalance, but rather a way to get rid of the weak ones. Perhaps there really was a reason for depression other than the imbalance and we humans ruined that with our laws, fears, and medicine.