Why do you stutter?

Why do you stutter? Today I discovered that the comedian Drew Lynch stutters because he got hit with a ball on his neck and went to sleep with a concussion, so this damaged some nerves in his vocal cords. I think or something. My stutter is hereditary. Out of all my siblings I am the only one who stutters. My mom has three siblings and she is the only one who stutters. My grandpa has two siblings and he is the only who has a stutter. Anyhow, I thought that this was really interesting and was wondering if any of you guys know why you stutter? Is it psychological, hereditary, accident, etc. If you feel comfortable and don't mind me asking.