If you have full-blown love (not just be slightly fond of, but really love) someone, you should continue to love her. It is a serious moral defect to be open to discontinuing one's full-blown love. This can be discerned from the phenomenology of full-blown love.
But a failure to continue loving someone shouldn't get one out of the obligation to love her. It would be "too convenient" if simply by doing the wrong of ceasing to love one were to get out of the obligation to love our beloved.[note 1] So our principle that if you have a full-blown love then you should continue to love can be strengthened:
- If you had a full-blown love for someone, you should love her.
But why is (1) true? I propose that the best explanation for (1) is:
- You should love everyone you can love.
The best alternate explanation of (1) is that love is relevantly like a promise: by acquiring full-blown love for someone one commits to an obligation to love. But this view is not plausible. Think of the way that children come to deeply love their siblings. This love can grow on them early, before they have the kind of moral responsibility that would make them fit subjects for undertaking lifelong commitments.
Now, we could stick with (2) as the conclusion. But everyone is in principle lovable. But perhaps not lovable by me? But an inability to love someone who is in principle lovable is a moral defect in me, though perhaps not one that I am culpable for. And moral defects shouldn't get one out of moral obligations. So:
- You should love everyone.
And that completes the argument. Definitely not a knockdown argument, but still something that should give some credence to the conclusion.