People stutter for different reasons. Not all stutters are the same. For some it's more emotional and than neurological. So these people report that they never stutter when alone nor when they sing. And then there's people who's stutter is more so caused by neurological differences than emotional or psychological. These people report that they stutter when when alone or talking to animals and even when they sing. And even then there's variation among these groups. If you want to know why we stutter, here's a video if you want the short answer: https://youtu.be/Wj8gz0Rp_GY?feature=shared The reason why you stutter even when alone is because your brain processes speech differently from other people. There's structural differences in the speech part of your brain so your brain is doing different things during speech. For example brain scans taken of people who stutter (pws) during speech reveal that occasionally the right brain is being activated during speech. Basically what's happening is the right side of your brain is recruited in order to help compensate for the structural deficits in the left part of your brain. But the right hemisphere of your brain isn't equipped for language and speech production so it doesn't work that well and causes an interruption or block in your speech. In non-stutterers the right side of the brain isn't being activated during speech because for a majority of people, language and speech are controlled by the left side of the brain, namely brocas and wernikes. Basal ganglia also plays a role in speech because it's responsible for rhythm and sequencing the speech commands. And apparently this part of the brain also isn't working properly in pws when it comes to speech.