Blocking oxytocin disrupts the pair bonding that is here a surrogate for love, and makes the voles more restrained in their emotional expressions. Conversely, inducing an excess of oxytocin in other, non-monogamous vole species blunts their taste for sexual adventure. In humans, though, the effects are much less dramatic – a subtle change in the romantic preference for the familiar over the new. So oxytocin is far from proven to be essential to love.

Of course, even if we could identify such a substance, any message – chemical or otherwise – needs a recipient. So where is the letterbox of love in the brain? And how is the identity of the “chosen one” conveyed, given that no single molecule could possibly encode it?

When romantic love is examined with imaging of the brain, the areas that “light up” overlap with those supporting reward-seeking and goal-oriented behaviour. But that parts of our brains are set ablaze by one thing does not tell us much if they are just as excited by a very different, other thing. And the observed patterns of romantic love are not that different from those of maternal bonding, or even from the love of one's favourite football team. So we can only conclude that neuroscience is yet to explain this "head over heels" emotion in neural terms.

Do we simply need more experiments? Yes, is usually the scientist's answer, but this assumes love is simple enough to be captured by a mechanistic description. Each reproductive decision can be neither simple nor uniform, for we cannot be allowed to be guided by any single characteristic, let alone the same one. Universally attractive though tallness might be, if biology allowed us to select on height alone we would all have gigantism by now. And if the decisions have to be complex, so must the neural apparatus that makes them possible.

While this explains why romantic attraction must be complex, it doesn't explain why it can feel so instinctual and spontaneous – unlike the deliberative mode we reserve for our most important decisions. Wouldn't a cool, detached rationality be better? To see why it would not, consider what explicit reasoning is there in the first place. Evolving later than our instincts, we need rationality only to detach ourselves from the grounds for a decision so that others can record, understand and apply it independently of us.

But there is no need for anyone else to understand the grounds for our love, indeed the last thing we want to do is provide others with a recipe to steal our object of desire. Equally, in ceding control to recorded cultural practice, evolution would place too much "trust" in a capacity – collective rationality – that is, in evolutionary terms, far too young.

It is also a mistake to think of instinct as simple, and inferior to careful deliberation. That it is tacit makes it potentially more sophisticated than rational analysis, for it brings into play a wider array of factors than we could ever hold simultaneously in our conscious minds. The truth of this stares us in the face: think how much better we are at recognising a face compared with describing it. Why should the recognition of love be any different?

Ultimately, if the neural mechanisms of love were simple, you should be able to induce it with an injection, to extinguish it with a scalpel while leaving everything else intact. The cold, hard logic of evolutionary biology makes this impossible. Were love not complicated, we would never have evolved in the first place.

That said, love – like all our thoughts, emotions and behaviours – rests on physical processes in the brain, a very complex interplay of them. But to say that love is "just" brain chemistry is like saying Romeo and Juliet is “just” words – it misses the point. Like art, love is more than the sum of its parts.

So those of us lucky to experience its chaos should let ourselves be carried by the waves. And if we end up wrecked on the surf-hidden rocks, we can draw comfort from knowing reason would have got us no further.

This article is part of Life's Big Questions, a new series by The Conversation that is being co-published with BBC Future. It seeks to answer our readers' nagging questions about life, love, death and the Universe. We work with professional researchers who have dedicated their lives to uncovering new perspectives on the questions that shape our lives. If you have a question you would like to be answered, please email either send us a message on Facebook or Twitter or email bigquestions@theconversation.com

Parashkev Nachev is a professor of neurology at University College London.

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