So him being Jecht's son isn't reason enough? Seems a good enough reason to me. The story is told through his eyes, anyway.
I really don't understand how people can complain that he's self-centered. Yeah, he is, but you're complaining about that? Are you sure you played VII and VIII? Besides, it works for Tidus's character - he's supposed to be a little douche, at least at first. And he's socially oblivious and blunt, which is a nice departure from protagonists like Squall and Cloud.