Jack just had that charisma that some people have that brings people to them. And, he has the habit of being standoffish when somemone new approaches him until he is ready to open up to them. For example, he doesn't approach Lureen -- he waits for her to come to him. I'm sure she was the one who wanted to dance, and I'm sure she's the one who started things in the car. When Randall began to make passes at Jack, he sat on the bench stoically listening to Randall, but never responding -- until he was ready. He didn't give Randall any positive indicators that he was interested in him and listening to his ideas about their going fishing together, etc. IN my opinion, Jack had to feel comfortable before he let down his guard, but once he did, he gave it his all. He took Lureen up on her matin' call- maybe because he was lonelyl; maybe beause she was the daughter of a rich man and he was ready for some of the good life for a change --, and he pursued Ennis with strength and purpose until he got what he wanted from him. Yes, I think Jack was a bit of a user, but not in a bad way. He just saw what he wanted after a long, slow look, and then went after it. Because of his personal charisma, he usually got what he wanted.