Being annoying is a very effective way to get under someone's skin, and it's especially so when it's done intentionally. Whether it's a repetitive sound or action, or whether it's actually done to someone or just in their presence, it's almost as distressing as actually causing physical injury. At its most innocent, it can be just done to pester someone, usually for attention or ribbing, and can also be a very effective way of getting what you want. Sometimes it's done to get a kick out of the target's discomfort