Well, it seems to me that the best relationships - the ones that last - are frequently the ones that are rooted in friendship. You know, one day you look at the person and you see something more than you did the night before. Like a switch has been flicked somewhere. And the person who was just a friend is... suddenly the only person you can ever imagine yourself with.

This quote is suggesting that some of the best relationships are the ones that begin as friendships. It states that sometimes, without expecting it, you start to see the person differently one day and you feel like you could be in a romantic relationship with that person, even though they were only a friend before. This implies that it is possible for friendship to grow into something deeper and even romantic.