The ending is ultimate irony. Tidus starts the game helping Yuna without knowledge of what happens to summoners. He then spends all of his time trying to find a way for her to survive, primarily because he wants them to be together after Sin is defeated. However he learns that if Sin is destroyed forever, he will be the one to die instead. It's a complete role-reversal. In the same way Yuna accepted her own death to save others, so too does Tidus accept his own death to save the woman he loves. It's so delightfully tragic and beautiful that I must confess that even I, a well known hard-case/nut to crack cried openly during the ending.