We really don't know. I know, for me, it had something to do with external judgment. We are fluent when talking to ourselves or to a pet but anywhere else it's a crapshoot. I suffered from an abusive childhood and began stuttering at the age of 5. I know I had a hard time not feeling judged by my parents for even being ‘imperfect’ because I stuttered. No speech therapy or warm fuzzies, only marginalization. I survived and at 70 became fully fluent through psychotherapy. My stuttering was caused emotionally and finally dealing with my cause, I recovered. Better late than never. Sue