Here is what I think is the reason.
Taken from this page on Cognitive Dissonance.
http://en.wikipedia.org/wiki/Cognitive_dissonance
Check it out,,,
The more severe a task the person performs for a group, the more interesting or worthy they must reason the group is to prevent dissonance.
Note: I made an edit below (**) and replaced "sever" with mild, as they had "severe" in both cases.
NJY
Effort justification paradigm [ edit ]
Further information: Effort justificationDissonance is aroused whenever individuals voluntarily engage in an unpleasant activity to achieve some desired goal. Dissonance can be reduced by exaggerating the desirability of the goal. Aronson & Mills [17] had individuals undergo a severe or mild "initiation" in order to become a member of a group. In the Mild (**) -initiation condition, the individuals engaged in an embarrassing activity. The group they joined turned out to be very dull and boring. The individuals in the severe-initiation condition evaluated the group as more interesting than the individuals in the mild-initiation condition.
All of the above paradigms continue to be used in fruitful research.
Washing one's hands has been shown to eliminate post-decisional dissonance, presumably because the dissonance is often caused by moral disgust (with oneself), which is related to disgust from unsanitary conditions. [18] [19]