My current educated-just-enough-to-get-me-into-trouble guess is its a timing problem. (Note: This does not apply to trauma-based stutters.) Sometimes Parkinson's attacks the basil ganglia, and when that happens people can develop a stutter, and the basil ganglia has to do with timing, this is my understanding of one of the current theories: the brain asks the speech producing bits to create the speech, but the timing of it all gets messed up. So, what you have is the brain/body trying to do multiple things at the same time. You can't produce a "g" sound at the same time as an "s" sound, so things get fubar. If that same person is in a group all speaking the same thing, there is an additional external clue to the timing, so the brain gets that extra little bit of help it needs to figure the timing out correctly. This doesn't happen all the time, but it can, and does, happen. (yay) There has to be a bunch more to it (Like why don't I stutter when I read out loud? What is it about being with someone else?) but that's my understanding. As to why do WE in particular stutter, it because if we didn't it would be unfair and we would RULE THE FUCKING WORLD. I mean, other people have to get a chance to compete, right?