So
why MC feeling suddenly become depressed?
Maybe the typical Japanese Hikkomori strikes again?
or MC in the past betrayed by his peer?
Nah man. It's nothing as simple as that.
This is Major Depression - the very clinical definition. It's a chronic condition where your neurochemistry is such that you keep slipping into depression no matter how good you have it.
It's a very insidious illness that is satisfied with nothing less than your total destruction. I know because I'm on a "OH NO YOU DIDN'T!!!" prescription of Wellbutrin and Remeron. (This is incredibly dangerous for some people because the Remeron slows down your ability to metabolize the Wellbutrin, so it stays in your system longer. And I'm already at the strongest "safe dosage" for Wellbutrin, too.)
And the hell of it? Some days, not even that is enough to keep the bad feels at bay.
This is what our man, Keigo, is going through right now. Only without the benefit of medication to act as a crutch.