Leah liked the left side. Whenever she took a picture with her friends, she was on the left; not necessarily on the far end, but just enough to notice. When she was with Sam? She was on the left. It was just something she had grown assumed to, and if she was on the right side of things Leah felt a nagging feeling tugging at her chest.
Jacob was addicted to being on the right side of things. He just couldn't help it; somehow he viewed it as a natural cause. So when he and Leah started dating, he would always take to the right side. Leah didn't really mind. She actually frowned a bit whenever she was on her right, which Jake found a bit funny, until she sneered at him and snapped a little, annoyed by his goofy smile directed at her. It was a good match, really. When he first saw Renesmee, she was on his left. And like Leah, he would feel a tugging pull at his chest, as if something wasn't quite right.
If only he could've figured it out sooner what was wrong…