In our society, defecation involves an individual in activity which is defined as inconsistent with the cleanliness and purity standards expressed in many of our performances. Such activity also causes the individual to disarrange his clothing and to 'go out of play," that is, to drop from his face the expressive mask that he employs in face-to-face interaction. At the same time it becomes difficult for him to reassemble his personal front should the need to enter into interaction suddenly occur. Perhaps that is a reason why toilet doors in our society have locks on them.
Erving Goffman