**WARNING!!! The author of this here post does not know if this has already appeared in some other thread, nor did he finish the whole of KHII yet. So please don't bash him... hard...**
Did anyone else notice that if you take away the "x" in Xehanort's name and mix up the letters a little (like you do with Organization XIII members), you get "no heart"? This would explain a lot. If he didn't have a heart from the start then, according to what dr. Finkelstein said during your second visit in Halloween Town, he longed for one, thus was absorbed in studying the heart, the darkness in it and the Heart of all Worlds - Kingdom Hearts. What's more, I recall Ansem the Wise stating in one of his reports that he found Xehanort without any memory. I think he might have already been a Nobody (but how could a Nobody become a Heartless and another Nobody - that I don't know). If I remember correctly, Roxas didn't remember his life as Sora because Sora stayed a Heartless for a very short period of time. What if the same happened to Xehanort (although it would be weird if a guy was called "No Heart").