I have been an SCA member since '98. I am addicted to the camaraderie, friendship and sense of family that exists living "The Dream" in the current Middle Ages.  Additionally, the competition, energy, exhilaration and act of learning and improving oneself that occurs during training, crafting and combat.
Nothing tops the feeling of standing, surveying the field before a battle, anticipating the coming test, except maybe the reminiscence after that battle has been fought and recounting the great acts of valour that one has witnessed from his fellow warriors.