Human flourishing includes a number of central goods such as friendship with God, friendship with neighbor, understanding of the world, appropriate autonomy, etc. Take any two goods, G and H, other than friendship with God. We could imagine an alien race of intelligent beings that are approximately as capable of humans in terms of goods other than G and H, but that normally are much, much more capable than we in respect of G and much, much less capable than we in respect of H. Maybe these are a race of individualistic scientists and philosophers who are barely capable of friendship with their fellows. Or maybe these are a race of very friendly and socially intertwined beings who are much less good at understanding the world. Call these aliens "xens".
There is nothing morally repugnant about xens, in the way that it would be repugnant to imagine a race of beings whose flourishing consists in causing misery to others (if such flourishing is possible). If we ever meet such a race of alien beings, the reaction we should have is "Vive la différence!" We ought not think ourselves superior to them and they ought not think themselves superior to us.
But now imagine that there were a form of neurosurgery that greatly increases one's capabilities in respect of G at the cost of one's capabilities in respect of H. It would be clearly wrong to perform this surgery on one's child, and it would be dubious if it were appropriate to have it be performed it on oneself. Why? If the life of the xens is no worse than ours, but merely different, what's wrong with such surgery? I think the obvious answer is: The life of the xens is great--if you're a xen. But we are not xens, and their life isn't for us. For the xens, less of H is normal. For us, less of H is abnormal.
So, thinking about xens and neurosurgery suggests we need a notion of normalcy. And this notion had better be morally significant. Merely statistical notions of normalcy lack moral significance. Why be like everyone else? (One might worry that being different from others leads to misery. But I deny this. My childhood was a very happy one, but I was very different from others.) This suggests a natural law intuition: a life with the particular pattern of emphases among pursuits that is characteristic of normal humans is something that has a moral call on us, unlike the pattern of goods embodied in the life of xens.