Within a shared dream one person must take on the role of the Dreamer.
The dreamer's role is to construct and maintain a dream space. Often the dreamer will add personal touches to his or her own dreams. A dreamer's base desires, like the need to go to the bathroom, can unconsciously affect the dream.
If the subject's subconscious ever suspects that he or she is in a dream controlled by outside forces, then the projections will seek out and kill the dreamer and any other foreign presence.
While other sleepers can manipulate reality in a dream to some degree, only the dreamer can adjust the architecture of the dream.