Emotion

"Vulcans, as a matter of custom and policy, suppress or think past all emotional influence by living lives of rigid emotional self-control through meditative techniques and training of mental discipline. Vulcans are not depicted as having no emotions; although they themselves make this claim, Vulcans are an exceptionally emotional people. They developed techniques to suppress their emotions precisely because of the damage they can cause if unchecked. In one episode of Star Trek: Voyager, Tuvok explains that Vulcans' natural emotions are "erratic and volatile"; if Vulcans do not strongly repress emotions, they can get violently angry in an instant. T'Pol once stated that paranoia and homicidal rage were common on Vulcan prior to the adoption of Surak's code of emotional control. In the original series episode "The Savage Curtain", Spock meets Surak and displays emotion, for which Surak reprimands him, and he asks forgiveness." From Wikipedia