Despite all of Tidus' insecurities and the battles going on inside of him, Tidus is oddly enough a very balanced person, because he finds out something that would make me go insane, and he stays calm and accepts his fate. What he finds out is incredibly chilling, and is one of the best parts of the game. What he finds out is that he is a dream. He, his father, his mother, Zanarkand, and all the people in it are figments of a dreamer's mind. And that dream is about to end.

It has to be the weirdest news anyone has ever received, yet Tidus almost instantaneously accepts it as truth. Not only that, but he dedicates the remainder of his life to helping Yuna defeat Sin knowing that once that task is done he will be gone. Furthermore, he decides to sacrifice himself as a final summon for that cause, much like his father did before him.

Tidus had already lost everything important to him and Yuna was the only thing he had left to live for. In fact, in his final moments, the thing on Tidus' mind is how he never got to show Yuna Zanarkand.

I think his sacrifice wasn't brought on by what he found out, but to help Yuna. I think that if he never found out he was a dream he would have still decided to become the final summon.