My introvert-ness comes from separation anxiety and social anxiety. both have prevented me from speaking up, making friends, and simply making any basic connections. for a while my parents thought it was autism (they tested me twice bc they thought the first test was incorrect 
); but it turns out i’m just a ball of stress with some seriously bad OCD 
So, i think it started from birth for me. I was just hard wired to keep to myself. social situations push me way past my mental capacity and usually end with a panic attack or something of that sort. I keep to myself and prefer it that way; less emotional distress and more energy preserved for my other mental illnesses 