I wish I knew. I believe my great grandfather had a stutter, and others in my family. At the root I think it's genetical, but it has turned into a fear issue. My immense fear, shame and embarrassment of stuttering forces me to contort my mouth and tighten up do all kinds of weird shit, making it so much worse and lengthening the stutter. Almost everyone slips and stumbles on words, but most can move on and continue with fluency. I am not able to as easily get over my stammer, which I think is because of fear. It's the same effect as flinching or raising your hands before you're about to be hit (although I never had issues being physically abused, just an analogy). I think I have this fear rooted in my childhood, stuttering in front of classmates, and being picked on a bit. Back then, I think it was because of simple mistakes and perhaps me being a hyper sensitive kid, and very concerned with what others thought of me. I now avoid stuttering so much, by using buffer words, that my stutter is not really noticeable, but its effect my my speech is significant. I can't get through a single conversation without avoiding certain words. All because of the same fear.