We can't be certain if oxytocin causes loving feelings, or if loving feelings release oxytocin, or both, but the evidence points to a strong link between the two.

Oxytocin is essential for erections and sexual responsiveness. But climax itself isn't necessary to release oxytocin. During an encounter, oxytocin in brain and blood generally reach their highest levels at climax, but they drop immediately. It seems the "Big O" triggers the return of oxytocin to baseline levels and releases the hormones that induce penile flaccidity. That's right, within five or so minutes of climax, your oxytocin is back where it was before you got frisky. 

It's thought that only brain oxytocin affects loving feelings, and it can't yet be measured in humans. (Oxytocin in the blood doesn't appear to cross into the brain.) Nevertheless, research on humans does show that "oxytocin-type" activities increase blood flow in the brain's oxytocin-releasing areas. Blood oxytocin, on the other hand, can be measured. It appears to cause the orgasmic contractions that help move semen around among other jobs.

Other pair-bonding species rely on more than jollies

Even assuming oxytocin is also what makes you feel totally In Love at the Big Moment, its immediate drop-off is not great news for those trying to stay in love. As it recedes, the desire to cuddle often goes with it. Somehow orgasm inhibits the "bonding chemical" in many lovers.

Perhaps the drop-off is why pair bonders (including humans) rely on more than just climax to keep bonds strong. Pair-bonding species spend most of their "us time" engaged in non-copulatory, oxytocin-releasing (bonding) behaviors: Grooming, huddling together, tail-twining, or, in humans, comforting, soothing touch, kissing, skin-to-skin contact, eye gazing and so forth. Interestingly, pair-bonding monkey mates who engage in the most bonding behaviors have the highest oxytocin levels.

These activities, as well as many that are flirtier (including copulation without climax), have an advantage over climax: They don't trigger an immediate return of oxytocin levels to baseline. 

Frequent, comforting feelings are important in maintaining strong pair bonds. We only deepen our bonds when we feel safe. What keeps us feeling safe is bonding behaviors (attachment cues). The oxytocin they release relaxes our natural defensiveness (by soothing the brain's sentry, the amygdala, and stimulating good feelings in our reward circuitry). The more dependable the flow of oxytocin via daily bonding behaviors, the easier it is to sustain a relationship. In contrast, a passionate one-night stand allows lovers' innate defensiveness to snap back into place pretty much as soon as oxytocin drops after climax. The next day, when she doesn't text and he doesn't call, defensiveness naturally increases.

Keep in mind that both oxytocin and dopamine (the "gotta get it!" passion neurochemical) are necessary to maintain emotional bonds. Both unfortunately drop after orgasm. So even if you're doing your best to elevate oxytocin with a cuddle after the Big O, you may not be feelin' it. Your desire (dopamine) is also dropping—not to mention often your eyelids. Pumping up dopamine with even hotter action often feels like a solution, but climax causes another drop in both oxytocin and dopamine. (More on an oxytocin-sustaining approach to intercourse in a moment.)

End of the honeymoon

The neurochemistry of passionate love is transient in another sense as well. New lovers are jacked up on special honeymoon neurochemicals, which go far beyond the effects of oxytocin. For example, they have extra nerve growth factor and cortisol flowing through their veins. Dopamine-releasing areas of the brain are activated. Their serotonin is often as low as the levels of OCD patients—which is why lovers obsess over each other. In addition, odd things are going on with their testosterone levels: They're lower than normal in men during early romance, and higher than normal in women—bringing their libidos more into sync. Yet all these potent neurochemicals return to normal levels by year two at the latest. Once that booster shot wears off, cracks often appear.