Xehanort likes knowledge. He likes seeing sights that no one else has doing doing things that no one else has done. He's attracted to darkness because it's taboo, dislikes light because it's common, and wants too reach Kingdom Hearts because no one else has done so before.
Above all else, he hates being restricted. Destiny Islands was a tiny, boring place that limits ones growth and Xehanort hated it for that. As he grew up, he found that no matter what, even after becoming a wielder and a master, that there are always borders and restrictions telling him "no" in some way, shape, or, form. I guess it can be boiled down to him wanted ultimate freedom and creating a new world where he can do what ever he wants without limitations.