With stammering/stuttering usually true that talking to oneself or a pet is not a problem. People have a neurological predisposition to stuttering, then when it begins to occur around age 3-5, at some point embarrassment and anxiety make it worse. It is a problem of the conflict of approach and avoidance. When trying to avoid stuttering but speaking at the same time stuttering is likely to occur. When speaking to yourself you aren’t worried about stuttering and aren’t trying to avoid it. Then there is not the conflict that results in dysfluency.