The first one was actually an accident. As is so often the case. Life cannot always be explained using an Homo oeconomicus.
However, before you are in that situation, ROI calculations absolutely do apply. To give you an example - most people on HN that have children probably have 1-3 of them - and that's about as many as they wanted to have. But if you somehow happened to already have 10 children, would you then be selling 7-9 of them (or indeed any) to the circus? Of course not. You would love them, care for them and do the best you can, and you would miss each of them dearly if they were gone.
And yet, given any degree of control over the situation, you would probably try not to have so many. Why not? Because of some calculation - let's call it the ROI calculation. It is precisely the same calculation that keeps you from having 10 children that keeps others from having any children at all.