I think Ms. Proulx makes the moment of falling in love in the story pretty clear, at least from Ennis' point of view. Please especially note the last sentence of this passage:
They had a high-time supper by the fire, a can of beans each, fried potatoes, and a quart of whiskey on shares, sat with their backs against a log, boot soles and copper jeans rivets hot, swapping the bottle while the lavender sky emptied of color and the chill air drained down, drinking, smoking cigarettes, getting up every now and then to piss, firelight throwing a sparkle in the arched stream, tossing sticks on the fire to keep the talk going, talking horses and rodeo, rough-stock events, wrecks and injuries sustained, the submarine Thresher lost two months earlier with all hands and how it must have been in the last doomed minutes, dogs each had owned and known, the military service, Jack’s home ranch, where his father and mother held on, Ennis’s family place, folded years ago after his folks died, the older brother in Signal and a married sister in Casper. Jack said his father had been a pretty well-known bull rider years back but kept his secrets to himself, never gave Jack a word of advice, never came once to see Jack ride, though he had put him on the woollies when he was a little kid. Ennis said the kind of riding that interested him lasted longer than eight seconds and had some point to it. Money’s a good point, said Jack, and Ennis had to agree. They were respectful of each other’s opinions, each glad to have a companion where none had been expected. Ennis, riding against the wind back to the sheep in the treacherous, drunken light, thought he’d never had such a good time, felt he could paw the white out of the moon.
Good Lord that woman can write. Allan
