Japan is a nation of favors. Thus the custom that when you see someone, you thank them for the last nice thing they did for you. "Thanks for taking me to the bank yesterday," or "Thanks for dinner the other night."
Such things are covered in the very vague, but appropriate phrase: "Senjitsu osewa ni narimashita" (literally, "the other day I was taken care of by you.") This handy phrase can even double as a greeting.
Since all favors should be repaid in Japanese society, consider your mere arrival in the country as significant enough an event to plunge you into eternal debt. People will do a myriad of favors for you and if you're like most people, you'll be left wondering which people you can just say "Thank you" to and which people you also owe a return favor to. And how much.
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