Depression is a state with causes. Remove the things that make it persist and it will go. If you’re talking about a chemical imbalance that itself is abnormal and could only truly respond to treatment like drugs (e.g. like schizophrenia and lithium) then I agree, but most depressed people have “normal” brains that are in a currently unhealthy state, so I’ll avoid arguing exceptional cases.

I’m also not suggesting that hope cures depression, I’m suggesting that hope prevents suicide. Basically, why kill yourself if there is hope? Depressed people aren’t being rational but then neither are optimistic people, each with their own distortions of reality. However, they’re not delusional, you might call it a state of hyper-realisticness that actually distorts through the destruction of possible branches in reasoning. An optimist thinks “it’ll be sunny tomorrow“, a depressed person thinks “it won’t be sunny again, ever”. Neither requires reasoning for their position snd can easily dismiss any given, but it can be done. Reason with an optimist and you will deflate them and they will react badly, usually we avoid this because it has little upside. Reason with a depressed person and they will also react badly but the upside is that it may sow the seeds of their remission.

Regardless of all their defences, distortions and dismissals, if they can be convinced there’s hope it will keep depression from being suicidal.