Do you think there's a rule that you must return other people's kindness/help?

Some people who helped you(you didn’t ask) for something seem to think that you owe them something that you must give back in return. Just think about it if somehow people helped your problem,and later on they expect you to do something they want since they once/have already helped you for something(they even remind you of what they did to you).

That’s the reason most of the time I’ll never believe pure honesty when people help me(well,they don’t say they want something but soon or later they’ll demand something from you). Well I don’t force other for their honesty because I understand that there’s no free thing nowadays and there should be reward for your effort.

So,what do you think if someday someone treat you like that? Do you expect something in return for helping others? Can you refuse their demand despite they’ve done something for you? Who do you think is selfish here?