The point being, Leah and Jake start off not liking each other. They're together for convenience, nothing more. Then they begin to draw solace and strength from their similar pasts; they've both been hurt, badly, so they're able to comfort each other in ways that others can't. They understand each other, and they learn to trust one another and this helps them to begin to heal and move on. Though their conversation is still peppered with sarcastic comments and they still needle each other, beneath it there's a feeling of comfort and friendliness. They still get annoyed with each other, but they've learned to work around it.
Leah and Jake's relationship felt completely natural. It wasn't perfect or instantaneous; it crept up slowly and took them both by surprise. They respected each other despite the sarcasm, as evidenced in the way Leah defends Jake against his "best friend" Bella, and Jake (according to Word of God) makes Leah his beta.
I think it would have been awesome if they fell in love, and then Jake imprinted on Renesmee- but recognized that his relationship with Leah was much healthier than any romantic relationship he could have with the newborn, and resisted the imprint in order to stay with Leah. I would have forgiven Meyer for breaking her own canon with the Dhampir thing if she had done something like that.





