Actually, widget285's recent fic was pretty close to my interpretation of their younger selves. I think both Lee and Kara were very career-driven at that point - in the Academy, time of peace, world is their oyster and all that. Neither of them were looking for romance. Lee, especially, hadn't had the pain of losing a loved one to make him look more closely at and value what he had. (Kara, too - we'd need to know more about her childhood to be more certain, though.)

They didn't have the fate of humanity riding on their shoulders; they weren't as grown up. In a nutshell, I see them as being more idealistic back then - and it seems like Zak, with his pursuit of a profession that he wasn't really suited for, embodied that idealism most perfectly of the three. I think Kara would have been drawn to that, particularly given her childhood.