Do the gods love an action because it is pious, or is an action pious because the gods love it? That old chestnut is of course the 'Euthyphro problem.'
A crucial assumption in Plato's treatment of it, it seems, is that an act or condition of love is for something, and what it is for provides the reason for (explanation, account of) that love. We can therefore distinguish (i) qualities that we attribute to things as a consequence of someone's love for them, and (ii) qualities of things that are prior to the love that someone has for them, and which motivate and explain that love. Euthyphro effectively takes piety to fall under (i), but Socrates thinks he has shown that it falls under (ii), in which case Euthyphro's definition has been refuted.
Isn't Plato concerned that, if we did not grant his assumption, then love--and the gods, too, insofar as they loved--would be irrational? Just as, in the application of a word to a thing, there must be a reason why we apply a word to that thing but not to others (the logos kata ten ousian), so, in our loving some things but not others, there must be a reason, located in the object, why this is so. If there were no such reason, then our love would be something irrational and even apeiron. (Love which doesn't begin because of something in the object of love also won't stop, ever, because of something in the object. It's therefore without any limit.)
We might object to the language (that love should be 'limited') or even to the concept, but that this is the view of Plato and Aristotle, at least, seems clear. We apparently find exactly the same notion in the Symposium--love is motivated by beauty--and in Aristotle's ethical writings--in his presumption that there are three reasons for loving something, its goodness, usefulness, or pleasantness.
I was therefore a bit surprised to find, when I was studying the book recently, that Eros Unveiled by Catherine Osborne reaches something of an opposite conclusion:
There are two main claims that I have tried to defend. One is that the correct way to understand the ancient tradition concerning eros is to see love as inexplicable, in the way suggested by the motif of Eros the god of love with his arrows. In other words we are not to seek the reason why anyone loves another by looking for some quality that is admirable or desirable in the other, but rather to see love as occurring regardless of whether there are desirable features in the beloved. The second claim follows from this, namely that where desire or admiration of fine qualities occurs and is associated with love, it would be a mistake to suggest that the desire or appeciation was itself love, or was the motive that inspired us to love. Rather it makes more sense to see desire, and appreciation of what is good, occurring as a result of love, as the expression of the love that enables us to see such qualities as good and desirable. (219)What of the Symposium, and what, then, of Aristotle?
Osborne dispatches with Aristotle by saying that he is concerned with a different phenomenon from love altogether: his writings on philia are about 'cooperation' or 'befriending', not love.
As for the Symposium, Osborne points to Plato's personification there of love as daimones who are intermediaries between earth and heaven. What this indicates, she says, is that Plato thinks that we need love even to be turned enough toward beauty, in the first place, as to find it desirable. Thus eros explains our finding beauty attractive; it is not the case that we find beauty attractive and then as a consequence love it.
(There is no discussion of the Euthyphro in the book, understandably enough.)