“One good turn deserves another” or Good Karma ! That’s what this is all about.
When someone returns a favor (en français, un service … par amitié, par exemple), this expression applies. Suppose that, for example, once upon a time, you gave some of your children’s childhood books or toys to someone with small children … with no expectation whatsoever of any compensation … and then later on, he returns the favor … by helping you … or even tutoring your children in his specialty …. Why, I’d say … “One good turn deserves another” and deeper friendship is the real, positive result!