Kyon found out (analyzing the bugs he had put around Juno) that her motivation to get stronger and surpass her sister had considerably decreased, especially after she had lived for years in his civilized world, where she had other things to worry about. Besides, she had fallen head over heels in love with the Lovr...
If a soul had a good motivating factor, it evolved twice as fast, without taking into account the increased number and time of training sessions. Without proper motivation, Juno would be completely useless. If she didn't evolve at a faster rate, Kyon would gain at best one stage of cultivation, and it's the same as getting nothing. Therefore, Kyon decided that Juno needed a new incentive and became an object of her hate.