Where to start...
Well, when Syaoran Li first came to town, he was not very nice to Touya's sister, Sakura. Then he developed quite a crush on Touya's boyfriend, Yukito. Li even tried to compete with Touya for Yuki's affection on a couple of occasions. Then towards the end, Li has a huge crush on Sakura. Also there's that matter of Li setting Touya's arm on fire. Is it any wonder Touya has it out for Syaoran? Maybe that's the real reason Li runs away whenever he see's Yuki. He's afraid Touya's gonna have to kick his fourth grade butt! (j/k) Anyway...these two simply can't get along. In my opinion, they have too much in common.