We're all just organisms responding to factors outside of our control.
The available responses we have are determined by our environment and thus outside our control and which response our brain determines to use is based on the sum total of things that have happened to us - also beyond our control.
We watch our determinstic universe unfold before our eyes, the only possible way it could have, and then start blaming or applauding ourselves. As if a falling seed deserves scorn for where the wind took it.
Having a sad life is not the same thing as depression.
Depression is a condition that persists regardless. Bad bouts are often triggered by legit shitty things happening, but unlike non-depressives who can react appropriate to the scale of the shitty thing and remember that it's a temp problem, depression encourages black and white thinking.
Depression is knowing in your heart that this is the worst thing in the world, is because you're irredeemably broken, and will never change or get better. In reality, that's not true.
Like most chronic health issues, symptoms ebb and flow, so sometimes you can be talked down or see the light yourself. Othertimes you cannot.