(A/N: If you need a real/physical age for Renesmee, I'd say she's about 17 physically, which would make her about 5 or 6 real age. Enjoy!)
Ordinary
The reason Renesmee had chosen Seth was because no one expected it.
Of course, her whole family had told her that she could choose whoever she wanted, but Renesmee knew that they expected her to be with Jacob. They wanted her to be with him.
Renesmee didn't want to be perfect. She wanted a regular life. And a regular life did not include a werewolf who thinks that she's their soul mate. A regular life included love that isn't forced by some wolf-like bond between them.
Seth was more laid back than Jacob. Where Jacob was always reminding Renesmee to be careful, Seth encouraged her to follow her dreams.
Seth was more relaxed around Renesmee's family, where Jacob was very tense and rigid around the vampires.
Jacob may have been Renesmee's soul mate, but Renesmee didn't want a soul mate, she wanted actual love.
And Seth was the one that Renesmee wanted to love.
(A/N: Believe it or not, (not) I enjoyed getting into Renesmee's head to write this. She seems like she wouldn't want some forced imprinting "love," she would want to make her own choices about love. And that frees Jacob up for Leah! (Hint, hint.) Although I really do love Seth too much to pair him with the spawn. Oh well. He'll live. Let me know if you see any typos, and reviews are love!)