It makes me wonder if it was due to circumstance or down to his own character that made Ennis a loner. He gives the clues to having to depend so utterly on himself with the conversation about his brother and sister, in which he states 'no more room for me'. Makes me wonder if it was an intentional loneliness, a form of self preservation, no one able to leave him alone again as he was already alone. As he says 'you got nothing, you don't need nothing.'
Maybe that is why being with Jack those few times rocked him to the core, he didn't feel he had to be lonely when he was with Jack and it went against every defensive mechanism he had been practising for so long.
Your self-preservation theory makes a lot of sense, aimi,after losing his parents, then his sister, and his brother,he would've thought that it was easier not to get close to people, that way there's no chance of being hurt by them

Of course, Jack comes along and makes all the loneliness disappear, but Ennis ends up losing him too after the summer,and that loneliness is probably even more painful, cause he now knows what it's like
not to be lonely. No wonder he was so overcome when he saw Jack again after four years!

The scene from the movie that immediately sprang to my mind reading this thread, is when Alma is trying to convince him to move to town, and she (somewhat manipulatively, some might say), reminds him of his childhood "you don't want it to be so lonely, do you?" Ennis immediately thinks of Jack, cause the only time he wasn't lonely was with Jack, and when he says "it ain't so lonely now", it's not Alma he's trying to convince, it's himself,and he fails miserably, it's obvious he's thinking about Jack when he's having sex with Alma

As for the question about what springs to mind when I think of Ennis, it's the scene where he's holding the shirts after finding them in Jack's closet :\'( I'm getting teary eyed just thinking about it, the way he tries to summon up Jack's essence from his shirt, knowing it's all he has left of him, and that he'll never again be able to escape the loneliness now Jack's gone :\'(