Yukito has this mysterious ability to make people fall in love with him on first sight. First Touya, then Sakura, and Syaoran is no exception. The very first time Syaoran sees Yukito, his cute face gets all red and he runs away. Yukito doesn't help this infatuation by being so sweet and kind and helpful. He also doesn't feel the need to explain to anyone that he isn't really available. This leads to a small war between Syaoran and Touya, who never got along well to begin with.
So for a while, Syaoran and Sakura are rivals for more than just the Clow Cards! They both get him presents and fight for his attention. Eventually, though, Syaoran realizes that the object of his true affections isn't really Yukito but.....someone else. ^_^