There's an episode in S4 where Juliet has Flashbacks of her therapist, who's basically a bitch, so basiclly not a very good therapist... but anyway, there's a bit where she says "Well can you blame him, you look just like her..." What she means is, that she's saying to Juliet that Ben fancies Juliet because there was another woman before her, who looks alot, if not exactly like her. Well, this might have been said before, or i might just be stating the obvious, but lo and behold, i know the answer to this question!
When he gets shot as a kid by sayid, and needs blood etc, juliet looks after him and cares for him. And alright he was unconsious, but sometimes, apparently the sub-consious can hear things, and he's known her for three years, what with her being with DI that long, which means that he would recognise her voice... Far-fetched???
Ah well, the rest speaks for itself, he was obsessed with juliet because sub-consiously, he was rememinded of the "other Woman", who was the same woman, she just didn't know it yet... Simple if you think about it. But then i'm pretty sure that quite a lot of you guessd that already.