Humans have long respected a reciprocity norm: after A does something nice for B, then B is expected to do something nice for A. Yes, how nice a response, and how strongly or visibly we expect this, varies with context. For example, it depends on the prior relation between A and B, on who is aware of these nice things, on the relative costs to A and B of their nice things, on the kinds of nice things done, on if A was authorized to do their nice thing, on if A did their thing hoping for a reciprocal response, on if A could have or did propose an explicit trade, and on if B accepted such a proposal.

At one extreme we have legally enforced debts, which are excused only in extreme situations (e.g., bankruptcy), while at the other extreme we have only weakly enforced social norms. For example, often when two people in sequence enter two doors in quick succession, the first person holds open the first door for the second person, and then that second person is expected to hold open the second door for the first person. They won’t be arrested if they fail, and observers might excuse them if they seem to be in an unusual rush, or less physically able to to open doors. But otherwise observers may think tend to think a bit less of them.

The Hare Krishna religion once famously gamed this effect by offering flowers to passersby in airports, and then holding out bags for reciprocal donations. This worked, at least for a while.

Note that in many kinds of relations we can prefer that A and B have other motives for doing nice things for each other, besides the threat of censure for violating reciprocity norms. Even so, observers may still disapprove if they see a very lopsided relation, with one doing far more for the other than vice versa.

Note also that A explicitly asking B for a favors trade is not a strong requirement here, even for legally enforced debts. For example, hospitals can charge for the help they give to people brought to them unconsciously. Rescuers can charge for rescuing those who didn’t ask to be rescued. If while you were on vacation a contractor accidentally replaced the wrong house roof, and did your roof instead, you can still be forced to pay for it, if you benefited thereby. This happens under the ancient and well-established law against “unjust enrichment”.

All of which brings me to my father’s day theme: you owe your parents some grandkids. They did something very nice for you by creating you. (Yes, a few of you might be exceptions who were hurt by this, but only a few.) Yes, they didn’t ask you first, but they couldn’t have asked you first. (That will be different for ems, who can be asked before making a new copy.) And not begin asking first is only one of many factors that can weaken, but not usually eliminate, your debt.

Most parents did in fact hope that creating kids might lead to grandkids, and grandkids are one of things parents most often and greatly hope for from their kids. Yes, you might be excused if your parents were especially mean to you in other ways, or if having grandkids would be a special hardship for you. Yes, we might not want to make this debt legally enforceable, and yes it might be better if you did such things out of generosity or gratitude, rather than out of feelings of obligation. But even so, you do owe them grandkids, even if only a bit.

You exist only because of an unbroken chain of parents that goes back to the origin of life. I like to compare this to the “human chains” often used to rescue people drowning in the ocean. If you are in such a chain, you have an obligation not to let go of those next to you in the chain, as everyone after you may then drown. I tend to think that you have similar obligations to your chain of ancestors and descendants; if you don’t have kids, all of the chain after you won’t exist.

Yes, many people (falsely) think that you can’t have obligations to people who don’t yet exist. Which is why I like to point them to this obligation to their parents, who very much do or did exist. Most of your parents, and their parents, all the way back along the chain, wanted you to continue the chain. And you owe them this, at least a bit.

Added: In the case of the two doors, I’d say you have an obligation to put substantial weight in your decision-making on trying to reciprocate. But if even with adding that weight, it turns out that you are better off not opening that second door, you are excused. This is why we excuse those who are in a rush, or who have less physical ability to open the door.