let's say that in their past lives, they had good reasons to distrust each other.
i'm actually still deciding on one of the specific reasons for that, but i know generally why. it's all related to the main antagonists.
i guess, as a hint, i like my protagonists/antagonists to wear shades of grey in the good/bad department.
the teenage love and angst and denial thing is going to happen in THIS life (largely because i can't decide who's going to hook up with whom and who dies in the end).
so lay that on top of past issues, and...