yea auron says that Jechts dying wish was to watch over Tidus. So auron did. Then Auron says that when sin came to Zanarkand, he changed his mind. He says that he could see in sins eyes (?) that Jecht wanted Tidus to come to spira and have a shot at life. So Auron took him to spira.

As for ultima seraph's answer, i never thought of that. It makes Tidus sound like Spiras version of Jesus Christ. The fayths dreaming up Tidus so that they could sent him to spira and destroy sin, then die.

Merry christmas to all.