Ya know, playin' Jack on the Performance Thread ( I know its not the same as the book Jack or the movie Jack, but its as close as I can make it), I kind of have a little insight into his adolescence, not much, but the main point is like Scott said. He is bright, sensitive, and loving; he's also witty and has an upbeat view on life, for the most part. I would also have to say he has an innocent heart, and I'm not certain if you will know what I mean by that. He was more worldly than Ennis but somehow Ennis lived his life as though he could never have anything he wanted, and that was something that Jack could never do. He continued to live with hope and optimism except for those few moments when Ennis stripped it away from him, driving him slowly insane.
Jack tried to offer what he could to Ennis. He gave of himself fully and completely and had practically from the beginning. Ennis seemed to be just the opposite, and the few times when Ennis brings himself to give back as fully as Jack are those moments in the film which we audience members continually fall in love with: those few moments of perfect happiness and bliss in a world which says that we're wrong, as the song goes.
In one way, that is why I find it so sad to leave the Performance Thread as Jack. Jack still has so much to give and offer of himself in the course of events yet to follow, and a few perfect moments yet to live, when the drive toward them cannot be stopped by any pain or death. It is easy to fall in love with Jack, while channeling him, I will say that much, and I would not be surprised if there was some aspect of this falling in love in the writing of fan-fictions regarding him.