Depressed people detach socially from their support network because that feeds the depression, and any plausible explanation feels as true as any law of physics. All you have to do is pick one: "These people don't care about me/They can never understand/They are better off without me as I will just hurt them."
One way I describe depression is that it's a detachment from reality. Whatever reality you are actually in, depression bends and warps it to such a degree that mentally healthy people could not understand even when described to them.
I am fortunate in that I don't really suffer anymore; as I was "getting better" one thing I came to embrace was how little sense it made. For awhile I kept a journal, and if I read the entries from the bad times aloud, I'd be constantly apologizing for them and admitting that the feelings didn't make any sense. I intellectually understood the meanings of the words, but the emotions behind them felt foreign. I mean, I remembered feeling that way, but I couldn't understand why. As things improved, I was able to get to a place where I could just resign myself to being hopeless for no real reason, but with the understanding that, in my case, it would fade on its own.
That wasn't the "fix", but just an interesting step along the way.
If you are confident in having learned that humanity means kindness, virtue, ubuntu, you may begin to experience symptoms of satisfaction, comfort, optimism, interesting challenges, self-realization and a steady flow of rewards.
The opposite absorbable world lesson the cruelest kind of learned helplessness. Look what fear has done to my body [0]. You might see humanity as fundamentally anti-human, kindness as ironic, nihilism as the stablest philosophy, even if you understand that, logically, the conclusion of nihilism is bound to invalidate the conclusion of nihilism itself.
Folks often invoke "curiosity killed the cat" as a way to communicate "unenriched and empowerment-lacking people are prone to self-annihilation while in their search for harmony". What they don't do is follow the thinking to its conclusion: "But the cat was simply trying to gain root access beyond a seemingly impenetrable social inertia. The cat knew that the subjectivity of contribution to the outcomes of local events was far more rewarding than the dissonance of a depressive hedonism trap.
It's always unexpected - if you could expect you would always appropriately manage your negative expectations, you would call such periods of discontent "vacations."
Like a situation where you're trapped in an action-selection fixed-point of free-energy-dissipation-minimization.
Like a phantom limb, you realize that the psychic model/hierarchical system of values/your world-view ultrafilter/your "rosebud" you placed faith into, you begin to realize that the abstraction might not really have captured every partial ordering/relation/utopia you could have benefited from knowing sooner.