So why is depression an "opportunity"?
Depression FORCES you to face and re-evaluate your fundamental beliefs (I am not normal), habits (eating/sleeping/exercising), and past trauma (parental emotional incest, past break-ups). The seeds for depression were probably planted a long time ago through the environment in which you grew up, your genes, personality, and social circle.
Depression is the most painful and hardest trial in the whole of human experience, including cancer, quadriplegia, and torture. It is literal Hell.
The only way to get through depression is to face and resolve your hurts and issues, whether deeply ingrained resentment for your parents because of emotional incest and neglect, or a breakup.
These are your automatic thoughts - thoughts that occur in fraction of a second that go unnoticed and unchallenged - that were deeply ingrained into your psyche from childhood. “They fuck you up, your mum and dad.”
Once you go through all of this you will come out stronger, more attuned to your physical body and emotions, with a new skill set to face all adversity. You also learned what it means to be alive - you gain a new appreciation for all the things you are able to enjoy that you couldn't before and for how easy life is after depression. Not only this, but you acquire the freedom to live your life to the fullest. You gain the confidence that you can face any adversity and recover from it.