To Americans in Japan, some Japanese seem impolite because they do not thank clerks, doormen, taxi drivers, waiters, and cashiers. Japanese seem to take the services provided by these people for granted, but in America one thanks almost anyone who provides assistance. And you should select the appropriate level in each case.
From the most casual to the most polite, the levels of expressing gratitude are as follows.
Thank you very much.
That’s really very kind of you. / Thank you so much!
The first or second form is appropriate when the waiter hands you the menu, when the cashier says “Have a nice day” as you leave a store, or the taxi driver puts your suitcases on the curb. The third and fourth should be saved for clients, friends and colleagues when they take special trouble to do something for you. Using one form for every occasion makes it seem that all favors are equally important, and that is not the case.
There is one point in which Japanese and American manners are exactly alike: the longer the expressions of gratitude, the more polite they are!