The scent of pine increases the higher I climb. The color of the trees and wildflowers seems to deepen. As I approach 10,000 feet, I'm exhilarated but breathless.

The intensity of the experience recalls another ritual, the tradition of hand-copying 278 Japanese characters before being granted access to Kyoto's moss garden. Both exercises serve as a meditation, purifying and preparing the mind for the sacred experience to come.

As I hike, I start stripping layers. One, two, three, four, until I'm down to my T-shirt. I'm reminded of how a butterfly develops through four stages—from egg to caterpillar to chrysalis to the winged delight that flits around me. I consider the wonder of this metamorphosis and the even more amazing phenomenon of the butterfly's migratory cycle.

For at least 10,000 years, these monarchs—each weighing no more than a fifth of a penny—have traveled as many as 2,500 miles with pinpoint accuracy from the northern United States and Canada to reach a place none of them has ever been. They begin their journey in the fall and remain in Central Mexico all winter. It is now late February and I am here just before lift off—mating is getting into high gear.

I round the corner and the show begins. One butterfly flutters by, and then another and another. I watch as dozens float past, their shadows dancing like polka dots on the dry dirt.

The monarchs are everywhere. Hanging on the pines like flat Christmas ornaments and clumping like swollen beehives on the ends of branches. Some evergreens are so covered with butterflies that they resemble maples in the fall. Although a monarch weighs so little, overburdened branches can actually break from the cumulative weight, killing many of those aboard.

Standing on the path mesmerized—I am among about 150 million monarchs—I take in the sound. The whirring is soft, like rain.

A butterfly lands on my hip, opening and closing its wings in the sun. Then another lands on my stomach.

Strangers take photos.

"Es un macho," says my guide Astrid. "You can tell by the two black spots."

Slowly, the male starts walking up my shirt.

"Look out, here he comes!" says an onlooker, as the butterfly reaches my chest.

Before it gets too personal, the monarch takes wing. The onlookers laugh. I feel light, happy, as if I've just been kissed by nature.

Just then, several sweaty local men huff and puff up the hill with a stretcher. They deposit Ida, a 90-year-old from Gulfport, Mississippi, on a rock. She blinks as if she's just been dispatched to the moon.

"I've never seen anything like it," Ida declares in wonder.

One of the most striking things about being in the sanctuary is not the monarchs, but the effect they have on the people here. Young, old, male, female, local, foreigner—everyone is smiling, helpful, friendly. It's as if the butterflies are bringing out the best in human nature.

On the walk down, I'm befriended by a pack of female athletes on their annual visit to the sanctuary from Mexico City. Two of the four carry dogs in their backpacks; a white poodle with a pink bow named Bianca and a Chihuahua named Merlina with two puppies.

I ask why these cosmopolitan babes keep returning to this quiet place when Mexico has so many playgrounds for the young and beautiful.