It's a complex topic, but after years of studying books and articles on stuttering and other psychological disorders, this is what I have come to learn. You must realize the following. What is speech? It's just sounds. But what are these sounds? These sounds are the building blocks of words, or at least they are now. In the early days, there was no such a thing as oral communication as it pertains to modern language. The sounds made by the early humans were solely used to express emotion, such as pain, excitement, anger, etc. So a person's speech motor system is strongly influenced and connected to his or her emotions... (If anyone wants to hear more, just tell me please. I don't want to spend my time writing this just for the sake of it. I have been stuttering as long as I have known myself. I have been studying the phenomenon of "stuttering" almost just as long, so if you have a question, I might be able to give you something to think about.)