That's a very important life lesson. Many people live unconsciously by a code of mutual obligation -- they will do something for you, motivated by feelings they would identify as generosity, expecting nothing immediately in return. It's rude to turn them down. They'll be hurt and feel rejected. At the same time you need to be aware that it's incumbent on you to return the favor eventually, in some form. I've seen a lot of, on the one hand, resentful feelings and on the other, baffled incomprehension, result when people who live by this code interact with people who don't.