In the first book, Max kisses Fang. I think Max always had feelings for Fang. She just never realized them until then. In the third book, Fang obviously likes Max. I think Fang had always liked Max too. He just never realized his feelings for her immediately.
My guess would be before the series started, that's why Fang acted like a jerk at the beginning of the series. Also, I have to guess because JP - and his terrible take on this story (only bashing JP not MR) - never classified or confirmed it.
When I went to a JP Q&A a few years back, he said that Fang loved Max since they were young, but didn't fully understand his feelings and didn't act on it until later, when she shared those feelings. He explained that at furst, he didn't fall in amor with her because she was pretty or funny, it was because he loved how leading just came to her.
am I an only person who think that the angel is... strange, odd and I don't like her! Since the first tom I thaught that there's something strange whith her... I can't explain it but I have never liked her :c