Rafael was attacked by a large dog when he was four years old. Fortunately, he escaped the attack without much physical damage, except for a scar on his left shoulder. However, Rafael has an extreme fear of all dogs and, as such, he avoids contact with any dog at all costs. He even moved from an apartment he loved when a next-door neighbor bought a bassett hound. According to two-factor theory, how did Rafael develop his phobia?