People have different origin stories. If I talk about myself, neither of my parents, nor of my grandparents stutter/stuttered. But interestingly there was a cousin of mine who stuttered. When I was a kid (3-6 years), I spoke fluently without any repetitions, but then I used to imitate my cousin (we lived very close by) whenver he stuttered and I still remember some of those memories of me imitating him. After a while, I started repeating the initial syllables of certain words as well like my cousin. It was almost like a contagious disease. In the initial years, it was mild and I didn't really care about it but then my parents, other family members and friends used to pinpoint my stutter which made me a lot more conscious and aware of it. There was an instance in my 6th grade, when my teacher made fun of me in front of the entire class upon me stuttering. After that, I became extremely hesitant of speaking situations (especially public ones) and stuttered severely (blocked) on every other word. It became worse till I was in my 10th grade. Surprisingly my speech improved a bit but I still used to struggle now and then. Moral of the story, neurons that fire together wire together and the only way to overcome stuttering is by hard core practice and exposure therapy.