Many mature reptiles should be isolated from each other, out of sight of other members of its species. And they shouldn't be allowed to view themselves in mirrors, which can provoke an aggressive display. To prevent painful bites, don't handle herps when they're basking and their body temperature is at its warmest. Some reptiles become more aggressive at the onset of sexual maturity or during breeding season. Some reptiles are naturally pugnacious. Tegus and monitor lizards are considered the most aggressive of the species.
For more information, please read the story When Your Reptile is Too Aggressive.