I feel it was attraction, friendship and love in that order.
The attraction and friendship were the basis on which the love could potentially grow. Love at first sight is a romantic notion. Indeed we seem to have true hopeless romantics here, but looks can be decieving, there had to be something about Jack that some one like Ennis can grasp onto other than good looks.. Jack was gorgeous, yes but what was it about Jack that could make Ennis feel this way and over ride his fear of attraction for another man?? Jack, a very giving person was generous with his *goofy,charmin,* down to earth personality. Jack, non-threatening and unspoiled, gentle and concerned.. Ennis had very low self-esteem and Jack was non-judgemental of that, giving Ennis more courage to open up.. Once Jack gained Ennis's trust, the walls came down.
Had Jack had a different personality, say arrogant , selfish ,homophobic, and overbearing, of course this would have never worked out. They did'nt clash, instead, the fit together like hand in glove. The chemistry, eventually off the page. Ennis falls in love with the essence of Jack, the person Jack was, once he got to know him. Jack's ability to make him feel that he was included in his world. The fact that Jack was a man, should have been a hindrance in Ennis's phyche. In stead he is drawn to Jack in large part because he ...is ...well just ..Jack. who happens to be a man. Nothing and no one else was gonna do. With Jack ,Ennis learned what true love was, but Jack had to openly show Ennis who he was first, in order for Ennis to fall in love with him.
I just don't feel that Ennis and Jack took one look at each other that day in front of Aguirre's trailor and said * I love that man*., or * I'm in love with him* Don't think it's that simple.

Sometimes loves has the element of surprise, Only after sufficient complementary doses of each other do I feel that, suddenly, they discover they're in love with each other. In SNIT I feel Ennis is seeking some kind of confirmation concerning what he was feeling. He finds it in Jack's eyes. To me Ennis's eyes are shyly searching Jack's for answers and discovers love there. could it be true? , Jack's eyes answer yeah, it is true Ennis ,guess what? Were in love and knowingly seals it with a kiss. The journey of discovery becomes complete and niether can deny that something special had transpired between them that is beyond mere attraction and friendship. This was something alot more deep. This is Love. 2 cent.