What a great topic! I've thought about this a lot too - what attracted them in the first place.
They were "each glad to have a companion where none had been expected." This came to my mind as well - they were both sort of thrown together for the summer, whether they liked each other or not, just the two of them in that beautiful but isolated environment, and having to rely on each other, so I always think they grew to love each other over time. If it had been in another time, another place, it might not have happened.
When they first meet each other waiting at the trailer, Ennis is just so closed off, fidgeting, almost intimidated. Jack is more outgoing and curious about this other guy he'll be working with, but I think he had a little bit of fear as well, only where Ennis pushes his feeling down and inward, Jack's are more outward, a lot of showy swagger and bravado, but they each share the same feeling, I think, both 19 years old, unsure of themselves, lonely, especially Ennis, trying to make their way in the world and already having received a few hard knocks in life. Ennis I think found Jack curious in the beginning, and is pleasantly confused by Jack's being so talkative, and Jack brings Ennis out of himself. It's such a huge turning point when he says "That's more 'n I spoke in a year." I don't wonder if Jack had another same sex experience or two before they met, because he seems a lot more comfortable with his feelings than Ennis does, or maybe he's just one of those people who goes with what he feels, even in such a repressive time. But he certainly brought out deep, strong emotions in Ennis, not just the intensity of their sexual feelings, but other emotions, like love. I think everyone can identify with that kind of youthful love, when it's given so freely, nothing you've ever felt before, and as Judy said above, they complemented each other too. I always think of a line from a song "a strong heart flows over, and empty heart fills." Just my opinion.
