During their first time on Brokeback, Jack made it clear that he needed Ennis. Everything that Jack wanted, Ennis gave to him. It was clear that Ennis would not have gone through the extra trouble for Jack unless there were some reciprocal feelings there. I thought the scene of Jack teasing Ennis with the lasso was very symbolic. Ennis wasn't even aware that Jack had flung the rope exactly in his footsteps until he became entangled in it and fell. By that time, Ennis had already fallen emotionally for Jack. It was just that Ennis wasn't cognizant of just how much, until after Jack left. I will never forget that pleading "are you gonna do this again?" look on Jack's face. It had to have taken all the strength within Ennis to deny that. And when Ennis collapses in a retching fit, shortly after Jack drives away, we see that indeed it did. That, to me, spoke of serious love most of us are lucky to get once in a lifetime.