SECOND INGREDIENT, PRIDE
Anxiety to cut a good figure before servants or allied classes of personal workers is a rich vein of pride which they do not fail to work for all it is worth. This kind of mind is always agitated from fear that the tipping has not been done handsomely enough. The satisfaction of having a fellow creature servile before your largess is a factor. The gratuity emphasizes your position in the social scale. It stamps the giver as a gentleman or lady. The smirking attentiveness of the servitor is balm to vanity.
Truly, if it were not for vanity there would be no tipping system.
THIRD INGREDIENT, FEAR
The power behind the tipping custom is Social Convention and the fear of violating it. The so-called social leaders, actuated by aristocratic ideals, establish the custom and the crowd follow suit in a desire to do the "proper" thing. The "what will people say" mania holds the average person in an iron obedience to a custom which is innately loathed. It makes you conspicuous to be a dissenter. The serving persons understand this psychology perfectly. To drift along with the current of social usage is easiest, whereas, to go against it requires the highest order of courage. The multitude simply rate it as one of the petty vices and let it go at that.