Even overthinking it too much when alone can cause you to stutter when alone. But the main thing is, if you do not stutter when talking to pets, or when talking to yourself loudly, then it's most like, in my humble opinion more an anxiety/fear problem which has lead you to develop a speech pattern early on in childhood, instead of it being some sort of unfixable illness or malfunction in the brain. I always used to think I had a unfixable disease in my brain, a malfunction, which caused me to stutter. Then I realized it made no sense anymore, if I can talk 100% fluently to my dog, to myself, to random other non-human things, but as soon as I have to talk to humans, I stutter. If it was some sort of brain malfunction because of some 'faulty wiring', it would make us stutter in ALL situations, not in a few select situations. Almost as if other humans around us possess some sort of magical power that suddenly makes us stutter. For me personally, it's all fear-based. Hope this helps a bit. I'm also just discovering as times goes by.