We only feel ticklish if we detect a sense of invasion or attack. If we try to tickle ourselves, our brains anticipate this "attack" and prevent us from feeling panicked. Strangely enough, even if a person knows that they are about to be tickled, the fear of being touched may cause the same "ticklish" reaction. Some people laugh even before being tickled.
我們只會在察覺受到侵犯或攻擊時才會覺得癢。如果試著替自己哈癢,腦部會自動預感到這種“攻擊”從而使我們感覺不到驚嚇。奇怪的是,即使有人知道他要被哈癢了,擔心被人觸摸,也會引起同樣“怕癢”的反應。有些人甚至還沒被哈到癢就先笑了。