It is heartbreaking to just say no - that seems a sufficient explanation. When we want to believe so strongly that we can't even stand to consider that it might not work, well that will create terrible incentives for people to actually make it work. So let's turn it around - what else can't we stand to say no to? How about parenting - we can't stand to not help our kids. Judith Harris says we have little effect on them, at least on how they act with others. And we can't stand to not send everyone who wants to college - that may not help much either.