What is Depression ?

Why would nature provide us with this depression response? Generally, the many processes going on in our bodies serve some positive, useful function. That is, they're adaptive, which essentially means that they are there to help us survive. My research leads me to believe that
depression is also adaptive—in other words, it's there for a reason. Think about all of the processes that go on in our bodies. They are there to help or to protect us. For example, our immune system protects us against such things as bacteria and viruses. Why shouldn't we also have built-in defenses against psychological attack? I am becoming increasingly convinced—this is the meaning of depression.
All right. So how can being depressed help us "survive"? This doesn't seem to make sense. When we're depressed, we feel terrible. But, we also feel terrible when our body is fighting an invading virus. Yet we know that we will get better and feel normal again.
But when we're depressed, we often don't even know why. Waves of depression can engulf us for no apparent reason. If our brains are really trying to"defend" us against something, then what is it? The answer to this question, I'm firmly convinced, provides the key insight to the problem of depression—and, I believe, a hidden strategy to overcoming depression step by step.