In today's society, one of the biggest persuasion factors (having the ability to convince people to make a decision) is reciprocity. I scratch your back, you now owe me, and you'll need to return the favor some day. It is a great trick when it comes to persuasion. But in the context of living according to nature, if I scratch your back, I'll scratch it ... and that's it. No desire for you to return the favor; no mental debit and credit accounting; no calling out the good deed.
Marcus compares other things that produce whatever it is they produce and how they just do it with no expectations. A horse designed to race, races, and that's all.
Just serve; act according to your social nature.
(see also Citadel p. 201-202)