Possible reason behind not stuttering sometimes?

Possible reason behind not stuttering sometimes? I don’t stutter when I’m talking to myself, talking to my dog, or reading out loud. I’ve realized that something all of these have in common is the fact that you aren’t replying to anyone or expecting a reply. I believe that not expecting a reply may somehow change your anxiety and stress levels, or it might change your brain somehow. Does anyone have anything to add to this theory?