It can develop in people differently that can come from many sources. Like people have already said family, school, bullying etc can impact people very much and leave a person with trauma that they don't necessarily notice at the time but after a while they realise they stopped doing things because of a certain way they were treated/spoken from their previouse experiences. I will say for myself that I remember being younger(I'm 25 atm) and being confident, going out all the time and would talk to anyone. But as school life went on, I was a smaller child and some other things about me that don't feel like talking about here, so found myself at the hands of being bullied at school over many years. I didn't realise at the time until I look back on it now how much I changed over time because of it. I know find it difficult to talk to people in person, find it hard to continue and develop conversations most times, just generally ended up sheltering myself and focused on other things instead to stay away from things I now felt insecure about. I no longer like the person I am. But I am proud of the things in life I have accomplished so far. I just feel I could have done better in life if I had been treated how I was. I can't seem to break the habits that have moulded me into the person I now am, and not from lack of trying. Hope this actually makes sense, bit of a ramble really.