I think it started as more of a curiosity like "why this lady dump me when she was just so obsessed with me?" The later on he sees how she has changed and starts to become more interested in her. And when he begins to like her he wants to treat her better. Since she always used to act in a way that he didn't like about her it took her dumping him to see that she became better.
I think he thinks like that at first but he seems open minded to people changing. Unlike most people, I think he observed her behavior more as a hobby of sorts trying to track what she's doing to win his affection then he realizes she really changed cause he's a good judge of character. He's smart enough to realize how she's changed by observing her. People can change and he knows that. First of all he didn't really hate her to start with he just he didn't feel anything towards her enough to notice that change at first.
Um why he suddenly head over heels for fl just because she acts different and dumped him and not to mention, drastically changed to a softie???