Idk why people can get really depressed because of one sided crush like that.

I think it was multiple things that made him this way. Today's chapter we see he has been enduring the last 4 years at his job taking sh*t from the older staff.

Seeing his only source of happiness, his crush, find that happiness he wants for himself without him was probably the last straw.

we know so little about jiri but sometimes people don't have anything to hold on to keep on living but one person who gives you a little bit of hope, so i understand jiri in that aspect