If you've read a few novels by James Clavell, you might come away with the impression that Japanese are highly motivated by family honor and would gladly commit seppuku to save face and avoid shame to their ancestors. While these concepts don't have much real play in Japanese life, one that does is giri, translated as "social obligation." If you receive a gift, you're expected to give an o-kaeshi (a return gift) that's equal to half the value of the original...this is why chocolate companies created White Day on March 14th, a day for men who received chocolates from females on Valentine's Day to return the favor. If you get invited to a Japanese wedding you're expected to bring a cash gift of $200 or more, but don't worry -- when you get married that couple will be obligated to attend your wedding, giving you back your $200, so no one has lost anything. The other day I went to the dentist to get a crown replaced so I could finally stop making Haganai jokes, and the wife of my dentist took me aside to ask me if I could find an English teacher for her. I didn't know of any good candidates and tried to decline her request, but she pressed a bag full of toothbrushes, dental floss and free samples of gum into my hands...and now I have a giri obligation to find someone to teach her English.
Marisa gives Reimu a Yukkuri as a White Day gift.