Last night I fell asleep to the sound of my neighbors arguing. I don’t really know them, though they’re friendly on the rare occasion Roscoe and I meet them in the hallway. They are never noisy. Until last night, they were a practically invisible presence in my life. But when I heard her crying, the kind of crying you only do when you think no one is listening, when you find yourself in that strange, desperate place between love and frustration, I felt…sad and a bit sick.

I’ve spent the summer blissfully removed from that feeling. Even as I have tried to remember, to write about the moment J and I finally decided to move apart, I’ve failed. The dialogue sounds tinny, the tone melodramatic. All I could come up with was a single, strange image: the moment the ball breaks through the glass instead of bouncing off of it. Something broken, something changed that can’t be repaired. Maybe I can’t tell the story, I thought, because it just wasn’t as momentous as it felt. Because the argument was barely an argument, just a disagreement about how we’d spend our Saturday. And maybe I shouldn’t explain it, I told myself, because in the end isn’t it always the mundane that drives us apart?

Though I couldn’t even hear their words, the tones of my neighbors’ voices validated the feelings which just last week had seemed so distant and overwrought to me. I lay in bed reading and trying not to listen, and thinking how grateful I was not to feel like that.

I spent yesterday afternoon reading about the brain. The research on the literal, physiological chemistry of love is too complex for a single blog post. Several chemicals and neural systems are involved and new research is always amending what we think we know about the mysteries of the heart, which in fact all reside in the brain. But understanding a little bit about it helps us to understand a lot about how love stories function in our lives.

Neuron, by Roxy Paine

First there are mirror neurons, the cellular basis of empathy–at least that’s the most popular theory at present. They appear to be far more complex than most brain cells, which respond to a single frequency, sound, or image. The same mirror neurons fire when we pick up a mug to drink from it, when we see another person drink from a mug, when we think about drinking, and when we say the word “drink.” Some scientists believe mirror neurons explain why we love fiction, and, by extension, movies, songs, stories of all kinds. So it’s likely then that when we hear a midnight argument and feel a kind of unbearable empathy, that’s our mirror neurons at work. They’re why when, in Dirty Dancing, Patrick Swayze says, “No one puts Baby in a corner,” we grin goofily, as if he’s speaking those words to us. They might explain why we love love stories: our brains feel temporarily like we’re the ones in love.

Which brings me to dopamine. I’ve mentioned it before, but usually in passing and never with the attention it deserves. It’s a pretty complex neurotransmitter, and while its functions are many, the important thing to understand when it comes to love is dopamine’s role in the brain’s reward/pleasure system. Bungee jumping and cocaine and kissing all produce dopamine. Stimulated by novelty and thrills, dopamine jolts us with a rush of pleasure, suppresses the appetite, energizes us, and incites cravings for more.

So while a good kiss may feel like pure, indulgent pleasure, our brains are actually doing important work. According to Sheril Kirshenbaum (who wrote The Science of Kissing: What Our Lips Are Telling Us):

A passionate kiss causes our blood vessels to dilate as the brain receives more oxygen than normal. Our cheeks flush, our pulse quickens, and breathing becomes irregular and deepens. Our pupils dilate, which may be the reason so many of us close our eyes. We also activate five of our twelve cranial nerves that spread out intricately to different parts of the face. The nerve pathways guide the way we interpret the world by helping us see, smell, hear, taste, and touch.

She adds that “our lips are associated with a disproportionately large part of the brain.” They are packed with nerve endings which are constantly receiving and transmitting information. Kissing is a way to test out another person, to see what kind of chemical response he or she might produce.

Like mirror neurons, there’s still a lot scientists disagree on or don’t understand in terms of how dopamine works. But most agree that its function, when it comes to love, is supported by norepinephrine, which focuses dopamine’s effects on a single person. Norepinephrine provides a chemical explanation for love’s cliches: sweaty palms, weak knees, fluttery stomachs. These chemicals, and a few others, are what make love feel intoxicating–it is, neurally speaking, like any other chemical high. And they explain love’s devastation: the absence of love looks like cocaine withdrawal to fMRI brian scans. (Kissing is also believed to produce serotonin, oxytocin, and adrenaline, chemicals we’ll have to get back to another day.)

This is the problem with kissing: You’d be a fool to do it. It sets us up for a physiological addiction. The more you get, the more you want. Over time, your neurochemical response weakens: you develop dopamine tolerance. And yet, it may be our most efficient strategy for gathering information about a potential partner. The longer you do it, the more spit you swap (to traverse from the parlance of scientists to teenagers), the more you discover.

In heterosexual relationships, kissing allows women to smell a partner, to make subconscious observations about his DNA. So giddiness and butterflies likely signal a man whose immune system is distinct enough from hers to produce robust offspring. When men kiss women or other men, they’re passing testosterone through the saliva, which increases libido in both partners, and explains why men are generally sloppier kissers (seriously, there’s real science on this).

I spent yesterday afternoon feeling rather enamored with kissing; it seemed so ingenious, such a profound function of our complex social lives. Kissing, I thought blithely, is about what’s possible. Then I heard my neighbors arguing and I remembered dopamine’s dispassionate havoc. It’s been so pleasant seeing love from the safe distance of stories, my mirror neurons flashing over good books and friends’ marriage proposals. It seems a summer of rock climbing and bike riding and beer drinking and sun provides an adequate alternative to love’s dopamine. But already the nights are getting cooler and the rain-cloud graphic threatens my weekly forecast. Labor Day marks the start of a new semester, a new season, and I feel my commitment to that easy, dispassionate pleasure waning (summer resolution dissolution) because it seems that understanding the danger of dopamine isn’t quite enough to dampen its lure.