Depression is a really big disease. Those who've had it before know just how debilitating it is. Those who've never had it before think they know what it is, but they really have zero idea.
Apparently, once you've had it, the likelihood of you getting it again is pretty high.
I personally think everybody should get clinically depressed for at least 12 months pretty early in life, say in their late twenties or thirties. There is no better character builder than this. They shouldn't get it too young because they may not have developed the resourcefulness to deal with it.
Being clinically depressed for an extended period of time but having to function anyway (go to work, pass exams etc.) is the ultimate survival course. It forces you to manage yourself. Once you come out of it, you are a different person: more confident in yourself, calmer, you talk less, you are less overoptimistic. You realise pretty soon that many of those you deal with do not have your realism and calmness.
If you are in it, you need to get out of it. Here's how.