Well in my family my dad is an introvert, my brother is and so is my sister. I am as well. My mom is an extrovert however. Basically all three of us are like our dad. My dad’s family were also mostly introverts (his father was one also however mother is an extrovert). I was one from the moment I became aware of my existence in the world. Basically I was born an introvert, I think. I believe genetics plays a huge role in determining behavior in addition to environment. Where I work part time there are so many extroverts and they always comment that I’m quiet or I sit alone and don’t talk to anyone. I just prefer not to. I like the quiet and like to spend my time deep in thought. I think I might come off as cold hearted to some people? Growing up I was also very shy which sucked. Volunteering at the hospital really helped with my people skills, I loved getting to know them and even though I don’t talk a lot, I can talk for hours with people that I really love 
becoming an extrovert would be impossible for me! So I don’t try to. Instead I will remain this way but battle shyness. The only downside is I get told to talk more. Some people assume because my mom talks so much that I do too 
P.S shy and introvert are not the same thing. There are non-shy introverts.