I remember it mostly being because I felt alone both at home, at school and in the world as though nobody understood me. I spent a lot of time reading about stuff I knew nothing about like politics, psychology, sociology and economics in search of a reason why. It made me have a very bleak outlook on the world like it was full of monsters just looking to eat me lurking in the shadows of things I didn't understand that affected my life. I was afraid and needed to feel tough. The bleakness I felt I was just projecting into anyone or anything I could. In a way it became part of my personality.
Looking back I could've realized sooner it was my fault for the isolation and general lack of belonging I felt. There was no one to blame but myself for pushing people away. It truly is a shame I spent so many of my teenage years being an angsty edgy prick at school while being my true self only after. I should've let more people see the real me.
I hope that answers your question OP. I'm getting too old to remember.