After eating in dozens of restaurants in countries where tipping is the norm, one figures out a pattern: Waiters, if one ever bothers to look at their expressions after receiving tips, are often never happy about the amount of tips received. It does not matter if one tips 12%, 15%, 18%, or 20%, the expressions are often completely blank or laced with a slight frown, indicating that the amount could have been more. They obviously cannot show negative attitudes outright, but the underlying unhappiness is all too clear.
Sometimes, those subtle expressions of unhappiness seems all too strategic. Tipping, after all, is not written in law, but exist as cultural norms. No written rules exist to stipulate how much one is supposed to tip when receiving what kind of service. Instead, for many people with doubts on what is appropriate in terms of tips given, the source of answer is usually friends and family who are more experienced. They learn tipping culture based on the norms undertaken by others they know.
Hence, if enough Waiters (or other service personnel who supplement their incomes with tips) show negative attitudes toward amount of Tip received, then customers will, over time, feel guilt about the amounts of tip they gave, and slowly increase tips. Cultural norms change, pushing tips higher and higher as service personnel continue to frown subtly no matter how much tip they receive (of course, abnormally large amounts like 40% or more would be excluded from the pattern mentioned above).
For instance, in the US, 10% tip was considered quite appropriate about a decade ago. Yet, today, whether speaking to service personnel or those who frequent restaurants, no one would think 10% is appropriate. Even 12% today is considered to be on the lower end, appropriate only when the level of service received was questionable and deserve some sort of subtle criticism. 15% has largely become the norm, with 18% fast becoming the "proper" normal amount to give in some establishments.
Some may argue that the increase in amount of tip given is rooted in the stagnant wages of service personnel and inflation that increase the cost of daily expenditure. But such argument ignores the fact that tip exist as a percentage of total bill, and food at restaurants, for instance, should (and quite justifiably) increase in cost as the cost of living increases. People cannot deny that ingredients for making meals and labor costs are both increasing, so eating out should necessarily be more expensive over time.
Yet, given massive competition in the restaurant business, many eateries are choosing instead to suppress the costs of their food at the expense of quality and employee welfare. More cheaper ingredients of dubious qualities are being used as substitutes for original ones stipulated in many dishes, while salaries for cooks and Waiters remain much the same. As such, restaurants have pretty much turned a blind eye toward their staff subtly nudging customers to pay more in Tips.
This subtle encouragement of employees seeking more tip is certainly unsustainable for the restaurants themselves. Whether or not good service was indeed received, clients do consider tips to be a significant portion of their expenses while eating out. Even if the meals themselves do not increase in cost over time, if tips continue to go up, so does the overall bill. The result is people paying what is essentially the same as more expensive dishes all the while getting uncomfortable looks from their Waiters.
A better alternative is to just increase the costs of the dishes themselves so the staff has less incentive to change people's perception of what is appropriate amount of tip. Clients are not stupid, they know what is on the menu is not the total bill they need to pay. But if they pay a higher cost for the food they get while getting happy looks for only paying 10% in tip, then there is double benefit for the restaurant in being perceived as both high quality in food offering and in having a friendly staff that is always satisfied with the tips they get.