And it's just as dangerous to assume that they don't.
In fact, evolution to sentience is often driven by fear. That organism that fears the giant predator capable of tearing it to shreds typically will live longer than one that doesn't, and that can be attributed to be fact. What makes the difference is whether the Shivans evolved naturally or not. If they did evolve naturally, then, (if the origin discussed in the Shivan Manifesto is accepted) it wouldn't be unwise to think that the Shivans greatly feared the travelers from normal space and avoided them at all costs. Once the species had lived long enough to achieve sentience, the fear would have spawned other emotions such as anger toward these invaders and a desire to stop the intrusion. When the Shivans finally became capable of communicating their intentions, they would have grown angry at the arrogance of the intruding species and wiped them out. Soon, the Shivans began to see other species enter subspace, and began to treat these new invaders the same way as they did the first species to enter subspace, completely annihilating them.
This is a perfectly logical argument IMO. It has it's flaws and feel free to point them out, but I feel that it is just as plausible as most of the other theories put forth.
