"Setting off" a stutter?

"Setting off" a stutter? My friend and I both have a stutter that we can control pretty well most of the time. However when I get too excited or energetic / passionate about something I tend to start stuttering. And sometimes, if we are in a conversation and I start to audibly stutter, my friends stutter becomes more prominent. Does anyone know a reason why this could be, or what's causing it? Any help is appreciated.