o1. He made the first move. Jack had his eye on Ennis from the first moment they met, and was actually damned methodical in luring him into their first sexual encounter. In the canon, while Ennis seems to be carried along in the whirlwind that is Jack and himself, Jack knows exactly what he's doing and is quite manipulative in that respect.

o2. Continuing from the point above, Jack likes getting what he wants. Don't be fooled by that guileless, amiable exterior. The boy can be juvenile and petulant when things don't turn out the way he wants them to. He continued to be obsessed with what he perceives as "fair" into adulthood and until his death. There are many examples of this in canon... Pretty much every time he speaks, actually.

o3. Ennis was not his first or last gay lover. Canon does not state explicitly that Jack had been with anyone before Ennis, but with the way he essentially stalks the other man and then manipulates him into their first encounter makes it pretty clear that he was no blushing innocent. Extrapolating from what I know of Jack, I believe he had probably done a bit of "fooling around" before Ennis, but it is possible that Ennis was the first to actually have sex with him, although honestly, the tone of the canon makes me doubt that, as well. Canon does explicitly state that from the time that Jack left Ennis that summer on Brokeback Mountain, he was having encounters with other men, and it was Jack's reckless handling of these that eventually led to his murder. This is important to note because while Ennis was only ever gay with Jack, Jack was just gay, period. It brings up a lot of questions regarding how much of their relationship was, essentially, Jack forcing the issue. Of course, it should be noted that Jack comes to TR immediately after leaving Ennis on Brokeback.

o4. That said, Jack was, is, only ever will be in love with Ennis. Although it's sadly been turned into a cliche with the advent of the film, "I wish I knew how to quit you" is original canon, and about sums up Jack's feelings on Ennis. Jack was obsessed with the other man from the time they first met. He spent a lot of time trying to put Ennis out of his mind, but it never would work. It was Jack and his aggressive nature that kept their relationship going all those years. While Ennis undoubtably loved Jack (some might say in a more honest, pure way than even Jack loved him back), it took years and years and years for the man to get past himself and actually seek Jack out, although by then it was too late.

o5. Jack loves a little dog. It's true. ;o) He has a soft spot in his heart for runts and outcasts, which is perhaps a reflection on how he feels about himself and his place in society.