Suppose that a neuroscientist is studying love, and she discovers that romantic infatuation is caused by high serotonin levels, while attachment is caused by oxytocin. Has she actually learned anything about love? More generally, what is the significance of discovering neural or hormonal correlates to particular human emotions or behavior?

Depends, of course, on what you mean by "love". Some philosophers and cognitive scientists distinguish between the (neuro)physiology of emotions like love and the phenomenal experience of love; others say that "they" are really the same, perhaps merely experienced from different points of view. If your neuroscientist is studying the phenomenal experience, then she has learned of a neurological correlate of at least one or two aspects of that experience. It's unlikely that she's learned how that neurological correlate gives rise to, or causes, the phenomenal experience, though some philosophers and cognitive scientists (such as the Churchlands) believe that eventually we'll cease talking in such terms (just as we no longer speak of diseases being caused by evil spirits). There is a famous (or infamous) thought experiment that is relevant to your question, but that reverses the roles a bit. You seem to have in mind someone who already knows about the phenomenal experience (in this case, of love)...