I've always thought this was an interesting angle to relationships, right from when I first read about them in romance novels. You see these two people who have been friends since high school, and who have been leading separate romantic lives for years suddenly or gradually develop feelings for each other. If they're lucky as is usually the case in these books, the feelings are mutual.
One thing I like about the idea of being friends before lovers is that it gives the two people involved the opportunity to get to know each other with less of the pretense people put forward due to instant romantic attraction that leads quickly to a relationship. Also, the best long-term relationships are those where the couple are friends in addition to lovers, where the couple will be able to spend long periods together without getting on each other's nerves.

