It is possible to "turn it off"
As you say, doing new hobbies, meeting people etc. They all trigger the "honeymoon phase". Your body literally creates chemicals that turn off any depression.
The key would be for depressed people to keep changing it up and finding things they like instead of sticking with the same.
The vicious cycle occurs because a depressed person doesn't bother looking for anything, so nothing changes, so they get even more depressed.
Even if they do become self aware enough in order to make a change (as you did), they still have the mindset that "I'm doing this because I am depressed".
This means they are still not enjoying the full experience of life, as they are predisposed to failure.
A depressed person requires a hard emotional trigger to "snap" them out of it, something unexpected, something that invokes basic instincts, something that even their depressed mind can't ignore.
A simple example could be triggering survival instincts invoked by physical pain - this causes the body to take over and force the brain to engage.
The brain is no longer catatonic - it is forced to deal with the issue at hand (survival). There is no room to "be depressed". It's act or die.
This invokes instincts that activate the adrenaline in the system.
After the danger is resolved, the body relaxes and there is relief, adrenaline high. The brain realizes "hey that wasn't so bad". This high is what needs to be exercised again.
This activates the mind to start finding ways to get that high.
TIL: Depressed people need to get their first hit on getting high on life.