When he first appears, Syaoran thinks of Kero as the impressive guardian of The Clow he probably learned about as a little kid. So when he first meets Kero-chan in his small form, he can't help being shocked and asking "what's this thing?!" Naturally, Kero-chan (who doesn't respond to insults very well) gets mad and frustrated to the point of tears… and then proceeds to bite the heck out of Syaoran's finger.
That event, compounded by Syaoran's initial bad attitude toward Sakura -- Kero-chan's chosen and beloved Cardcaptor - establishes Kero-chan's (extreme) dislike of Syaoran. He refers to him only as "gaki" ("brat"), and rejoices whenever he's not the one to seal a Clow Card. Syaoran, in turn, never misses an easy chance to diminish Kero-chan (which gets his fingers a good biting more than once).
While both Syaoran and Kero are proud and impulsive by nature, their relationship does improve during the course of the series. Although Kero-chan is not one to forgive and forget when his pride's involved, he grows to respect Syaoran once he stops being Sakura's rival, and Syaoran also comes to respect Kero-chan's abilities.
Kero-chan and Syaoran's relationship adds a nice amount comedy to Cardcaptor Sakura, and in a way Syaoran brings out the best in Kero-chan.
