They are professional magicians, and their guilds were at war at the time. Not long after that Gajeel was actually saving Levi's ass and has been ever since. I don't think there's anything overly peculiar about it all. Back when he kicked her ass, they were only strangers doing their jobs, nothing personal there. The personal feelings developed later.
Also, Natsu must like Lucy far more. He's always hanging around her, actively seeking shared missions with her, invading the privacy of her home without an invitation, etc. He barely meets Lisanna compared to all that. He's just such a kid he doesn't do anything about it.
I think Fairy Tail has a fair number of successful couples (or what even non-shippers could feel like are potential couples). more than most long-running shounen.