The rule is very simple. First, the source gives you something. Once you accept it, you are now obligated to give something back. Note that you are not given a reward, because rewards are given for something that you have already done or will do. That first something given by the source is yours without you doing anything in the past or the future to earn it.
Reciprocity operates in many social relationships, especially with visits and dinners.
For example, a new couple moves into the neighborhood. You invite them over for dinner. Now, the new couple is obligated to give you a dinner in return even though you said nothing about it. I know some people who will refuse that first invitation because they do not want to get trapped into the spiral of reciprocity. When You Take A Stand, You Should Be Consistent.