Some kinds of roles succeed in imposing norms on those in roles of that kind. For instance, friends should help each other because they are friends, and spouses should be faithful to each other because they are spouses. Other kinds of roles do not succeed in imposing any norms. If I hurt you, I cannot rationalize my action by saying that I did that because I was your enemy. That may be an explanation, but being an enemy imposed no norm on me. At most it imposes merely apparent norms (before Socrates, the Greeks thought you should treat your enemies badly--they were wrong). What makes the difference? What gives some roles the genuine normative power they have?
Here is a natural law hypothesis I am attracted to. There are some natural human roles, and they each impose norms on those who fill them. I've in effect argued that spouse is one of those roles. Some other plausible examples: parent, child, friend, authority, subject of authority. Our nature specifies a potentiality to such roles, and gives them their normative power. There is a classificatory hierarchy between the natural roles. Spouse is a sub-role of friend, for instance. (An interesting and controversial question: are husband and wife natural sub-roles of spouse? If so, there will be further norms to being a husband and to being a wife.) And then all roles that have normative power are either natural roles or sub-roles specialized on the scaffolding of one or more natural roles, inheriting all their normative power from the natural roles. For instance, the roles of president and monarch are socially constructed sub-roles of authority, and their normative power over those in the role entirely comes from the role of authority. What about the normative power of the role over other people? That I suspect actually comes from the other people having the roles of citizen or subject, respectively, which are both sub-roles of the natural role of subject of authority. Being a monarch spouse is, on the other hand, a constructed sub-role of two different roles: monarch and spouse. (Think here of object oriented languages with multiple inheritance.) And while spouse is natural monarch is a constructed sub-role of authority.
In the above, I was talking about roles considered as general types, like spouse or parent or monarch. These types have tokens: spouse of Bill, parent of Joey, monarch of Canada. One can think of these token roles as also sub-roles, specializations of the role. An interesting question: is there any token role that is natural? That would be a token role whose normative force comes not just from the type role that it is a token of, as when being a spouse of Bill gets its normative force from being a spouse of someone-in-general. I think one plausible case is when the role is with respect to God. Being a subject of God may be a natural role. (What about friend of God? Perhaps that, too, but there the relevant nature may be a grace-nature.)
We can ask some interesting structural questions. Is there a highest level natural role? Perhaps being human or being a person. I have in the past speculated that it might be friend, but I now think that's mistaken, because friend roles are tied to particular individuals, while some of the other roles are not: an authority need not change qua authority when subjects are replaced by others (through conception, death and migration), but a friend does change in respect of friendship when friends come and go.
And how is this all tied to love? I suspect like this. Love isn't itself a role. But the roles determine which form one's love should take. Maybe in fact that is how they exercise their normative force, and that is what explains why there can't be a natural role such as enemy.