I think this is a really good question, and one I hadn't thought of before. Yes, of course, Jack has good looks and charm... presumabley, though, so do others, and Lureen seems to be a girl who could have anyone she wanted.
Still, maybe there's no real mystery as to why she was attracted to him initially. Possibly he did really stand out in that setting. But then, why did that attraction lead to marriage? Almost anything is possible:
1. Was it just a daliance during which she got knocked up?
2. Was she trying to rebel against daddy by choosing someone daddy would hate?
3. Did Jack deliberately charm and ensnare her, because he figured if he couldn't have Ennis, he might as well be well-off and comfortable?
4. Was she attacted to Jack's vulnerability and sadness, missing Ennis and never really fitting in, and, being a dominant sort of girl, did she want to sort him out, take care of him and give him a good life because he tugged at her heartstrings?
5. Did she 'recognize' him as someone who was a bit like herself - fun and highspirited, and also truly kind and a good friend? Did she recognize that, though it might not be true love with fireworks, she could do worse than to marry someone she liked so much?
hmmm.... I prefer #5. #4 would be my second choice.. I hope it's not 1 or 2... or, especially, 1.5, in which Lureen was already pregnant when she met Jack and was looking for someone to marry to pass the baby off as his and legitimate? But then, she wouldn't have been barrel riding would she? Unless that was Plan A, lose the baby?
I think we just don't know... and we could write a whole episode of The O.C. about it!