The reason why we think there is a moral obligation not to create unhappy people is that the presence of this suffering would be bad (for the sufferers) and the absence of the suffering is good (even though there is nobody to enjoy the absence of suffering). By contrast, the reason we think there is no moral obligation to create happy people is that although their pleasure would be good for them, the absence of pleasure when they do not come into existence will not be bad, because there will be no one who will be deprived of this good. It is strange to mention the interests of a potential child as a reason why we decide to create them, and it is not strange to mention the interests of a potential child as a reason why we decide not to create them.