When Paul, my future husband, kissed me for the first time, it was sweet and sexy and sort of salty—the way a kiss by the ocean should be, if you had thought to imagine it that way.

We made a date to kiss the next day, which didn't come soon enough. We met again and again after that. We spent hours hunched over pitchers of beer or tall iced coffees, our ankles intertwined, our eyes locked, our faces just inches apart. Quickly, without thought or deliberation, we traded coffee shops and sports bars for his studio apartment. Our tussles were movie-star sexy; sometimes, we'd pop a button off a shirt in our hungry dash to undress. Paul and I tumbled through lust, and then into deep love. For us, there was nothing more than each other; our jobs, families, friends, and personal interests were steamrolled by an overpowering desire to consume each other entirely. We tapped every form of communication to express our passion, filling our days with a barrage of mushy emails, text messages, and phone calls. (If we'd had the means, we would have hired a pilot to etch "I WANT YOU" in the sky.)

Back then, my heart beat fast every day, triggered by the sound of Paul's voice or a glimpse of his name in my inbox. And I couldn't imagine how it would ever slow down. But two and a half years later and six months into marriage, things are different. Though we're still young—both 32—sex is no longer the driving force in our relationship, and the love letters have all but stopped. Sometimes, it seems as if the grand task of making a life together is more important than keeping the fires burning. And there are days when I wonder if we'll end up like our parents, who divorced when they were in their thirties. But most of the time, I feel certain that our deep feeling for each other will persevere, carrying us to retirement and beyond.

Before I place bets on the longevity of my romance, I have to ask: what, exactly, is this thing called love? So far, I've come to learn that as quickly as it strikes, it can also vanish—poof!— leaving nothing more than a lingering heartache. And it’s clear that love's reach is vast: most of us are looking for it, trying to maintain it, or working to get over it. But where does it come from? Where does it go? And can it last forever?

Helen Fisher, PhD, an anthropologist and Rutgers University professor, has made the chemistry of love her life's work. According to her, I'm right on schedule. She says the first stage of romance—the one with all the kissing and love letters—can last anywhere from one to three years. Would I have appreciated our giddy phase just a little bit more if I had known that it was going to end?