My take on this: When you are all by yourself and talking to yourself you don't have to prove to yourself that you can speak fluently. When you are around others we want to fight our stutter, and we want to speak fluently, because some of us see our stutter as an imperfection which we want to hide to the best of our ability, in this fight to hide it the stutter can become more severe cause we are only focussing on the not to stutter part.