There's also a conspiratorial in-this-together feel, like naughty kids, which forms a closer link betweeen them. In some ways scenes like this (and the wrestling one) are the sort of thing both probably missed when they were children.
Yes, yes, yes.

Jack was an only child living in the middle of nowhere, and Ennis had distant (movie) or abusive (short story) older brother who wasn't the type to bond with Ennis...
I'm on one of my hobbyhorses now, but I see part of the unique beauty of this movie as the chance we see for two human beings to experience an awakening to human connection. Jack had his mother, it seems, and Ennis had Alma, but that was it. (And I'm not sure that either of those relationships were ideal: Jack's mom may have come around more after Jack's death; and Ennis was looking for a way out of homelessness by marrying Alma). Certainly neither of them had a good, close, comfortable affection with another *male*, which is important for boys' development.
After the first night of drinking and talking (according to the short story), Ennis thinks he had "never had such a good time in all his life, and felt like he could paw the white from the moon."(quoting from memory.) I find that very touching. The sort of friendship that most of us take for granted, or, in a pinch, that we could strike up for just one evening, is the sort of thing that Ennis treasures.
Personally, I see "falling in friendship" as just as beautiful a thing as "falling in love." (In Greek this would be falling in "phileo" vs. falling in "eros.") Two different kinds of love, but both very beautiful. And Ennis and Jack got to experience both that summer.
And later, perhaps, the deepest love, agape....