There's currently a lot of really fascinating research being done into what causes stuttering. Though they're still learning about it, the consensus is that it's a neurodevelopmental disorder with a strong genetic component. Basically, speech is a really complicated thing . A lot of connections in your brain have to occur perfectly, all at the exact same second, in order to speak fluently. In people who stutter, this connectivity is faulty. Conversing with someone else -- taking in what they say, deciding what you're going to say in return and when to say it -- is a lot more complicated than just talking to yourself, which is why I think we struggle so much when talking to other people, but can often speak fluently out loud to ourselves or to pets. As we all know, anxiety can make it worse; but it isn't the cause. I'm not an expert, but I'd guess your brain goes into 'conversation mode' when you have to approach people, which makes it harder for you. Here's a really interesting podcast: https://stuttertalk.com/what-causes-stuttering-with-dr-soo-eun-chang-568/ StutterTalk has done a whole series on what causes stuttering, talking to many of the current experts in the field -- well worth doing a search on their site. Edited to add link