It is just their personality. I don't think anything in particular causes it.

For me it was being bullied as a kid, and not fitting in with the crowd. So it was easier to stick to myself, and do my own thing.

The internal reflection of one's experience with humans. Both the fear of getting burned again and the realization of one's own faults. Humility and experience.

born that way. nothing wrong with it.

Not knowing where they fit in.

Genes, anxiety...past experiences

bullying is one example low self esteem/belief overly strict parents.

rejection

maybe thinking about stuff quite a bit

Birth, in my case. 🤔

i was a very talkative child. Constantly being told to "shutup,you talk too much,dont talk to adults when they arent talking to you,go outside". I got bad report cards for talking too much. I got in trouble for being too chatty and bubbly. I was scolded and always told to stay quiet and shutup. Over time i did shutup, i did stay quiet. It changed my personality. And this was when i was 5, 6 maybe 7 years old. It has BECOME my personality. I was not born introverted

genetics. believe me

life

Birth

Just leave em be lol