I've come to see depression partially as a semi-stable brain-state with reinforcing feedback. Some of that feedback may be chemical while some is psychological (thought or feeling). It may even persist once the major stimuli that got to that state have been removed. As such, any intervention that pops the patient out of that state could be permanent so long as the other things that led there don't happen again. Or if those things are still present, any intervention will be short lived.
I can definitely see this being true. Likely why things like psychedelics can help but only for a few months. Also would explain why CBT works as well as it does. I actually wrote something on the looping nature of my own depression that was in the form of a loop.
Engineer/Programmer. I had a bout of depression some time ago and had an amazing time watching my own thought processes and reading about psychology. I hope to write about my experience some day.
