A lot of unfortunate circumstances
In elementary school i was a normie and everything was pretty great, i had good friends, kissed girls and even beat up the school bully at one point. But then i got to middle school and everyone treated me like shit. I was bullied for having a weird name, and for having a "weird" head-shape. I would be yelled at and called names by EVERYONE including chads, nerds, stacies, autists, illegal immigrants, etc. They would even hunt me down and physically attack me. The one person i called a "friend" treated me like shit but i couldn't leave him because he was the only person i had anything in common with.
My experience at middle school shocked me, so in high school i trusted no one and became a recluse. I only spoke to people i knew from elementary school, in hindsight this was a big mistake but at the time i was mentally scarred. The friends i ended up making in high school would be some of the worst people i ever met. They treated me like garbage and would mentally abuse me every single day while passing it off as a joke. This crushed my self esteem and made me think that there was something fundamentally wrong with me. Because of this i rejected all advances made on me by girls because i couldn't imagine a world where a girl would actually find me attractive. I was also bullied for being shy.
At home, i had no father and was raised by an extremely paranoid mother who would never let me leave the house or do anything social. Even up to the age of 20 she would never want me to leave the house and it was always a battle to go anywhere and do anything that wasn't related to school. This made me a nervous wreck whenever i was out because my mum would always pressure me to come home as soon as possible even if it was in the middle of the day. Eventually i just preferred to stay home all the time because that way i wouldn't have to fight with my mum. I even missed out on getting laid when i was 16 because i knew my mum would never let me leave the house randomly at 4pm. If my father was around things would have been a lot better.
Nowadays i'm completely lonely with no friends due to my lack of social skills. I'm currently in uni and no matter how hard i try to be friendly to normies, they can tell that my social skills aren't good so they immediately ghost me. I've tried coming out of my comfort zone but in the end i always get ignored texts and people pretending like they don't notice me when they see me in public even though we're in the same class.