It's the Martha/Mary thing from the New Testament, the one who runs round doing practical things and misses the message, as opposed to the one who sits and listens. Here Jack's the creative one, while Ennis re-directs his feelings into practical action. He does this throughout the film, and tries to substiute deeds for emotion - it's true that many people who have problems with expressing their emotions do the same.
The harmonica scenes, and the later one where Ennis says he's giving thanks for the peace and quiet because Jack forgot the harmonica are warm and intimate and completely asexual - these are two people who like each other, share jokes, have a history to which they can refer back... this ISN'T a one-shot thing, of course, it's far more.