The brain is a strange thing. As others have mentioned, some of us simply don't stutter by ourselves and seems to be only around others. Some of us don't stutter when whispering, yelling, singing or with a fake accent or voice. It's inconsistent for me with somethings tbh. Some people think the stutter is linked to something the person found "traumatic" but it varies for everyone. Even speech therapists don't know a whole lot about it because it's just something we don't know much about