what was it about annette that made sebastian fall in love with her? and why was katherine so threatened by it?

he fell in love with her because she proved she was more than just a challenge, she taught him how to feel, how to smile, how to have joy and happiness in his life. and kathryn was jealous because now that sebastian had annette, she was truly alone. and this is just my view on the whole thing but, i think kathryn was in love with sebastian, thats why it hurt her so much to see his journal in the end, probably why she was using coke (to deal with actually feeling love for somebody), and why she was so upset to the point she was ready to ruin sebastian for falling in love with annette. it all makes sense if you just pay attention and read between the lines(:

kathryn was so jeaoluous because seabstian found his first true love and she don't was that girl.
She loved Sebastian but he cahnge her mind and then she didi't love her.
She loved annete not only phisycally like kathryn
he fell in love for the first time