Yuna is very self-sacrificing, doing everything for the good of others. She wants to save Spira because she truly loves the world, even if it means that she must put her own life on the line. She smiles because she doesn't want everyone else to be sad - she hopes that by a show of optimism, others will have hope.

It is very rare to see Yuna indulge her own wishes. Even when offered the chance to abandon her pilgrimage, she refuses - it's her duty, and she cannot step away from something she feels so strongly about. She talks to Tidus about possibly stopping her pilgrimage, but eventually says that it is not something that she could do. "If I give up now ... I could do anything I wanted to, and yet ... Even if I was with you, I could never forget." To abandon Spira would be unforgivable. She would never be able to live with a clean conscience, knowing that she could have done something to ease Spira's suffering. Sure, someone else could defeat Sin, but if you want something done, it's best to do it yourself.

She doesn't go on her pilgrimage because her father was a summoner. She truly cares for Spira, for both her friends and people she's never met. To have made a difference in someone's life, whether they ever get the chance to thank you or not, is really worth it. Though her efforts would ultimately be fruitless because Sin would return, to be able to say that she was doing something worthwhile with her life is something that cannot be denied. Happiness is not necessarily an eternal state - sometimes, moments are all that we have and we have to hold onto them. Even though she will die, she knows that her sacrifice will allow someone else to live, and thus, she can go on and walk calmly towards her own demise. It is not that Yuna wishes to die, nor that she wishes to leave Spira, but she feels that her life is a fair exchange for a moment of peace.

I remember, when I was only seven years old, in Bevelle that day. My father had defeated Sin, and the whole town was out in the streets. Everyone was laughing. They all seemed so happy. If I defeated Sin, that would make everyone happy ... wouldn't it? I must do what everyone wants, not just what I want.

As it were, Yuna does manage to save Spira without sacrificing herself. In spite of her having saved the world from Sin permanently, she stil feels obligated to help out wherever she can, and that's basically the entire premise of FFX-2. Even though she doesn't need to help out anymore, and even though everything she helps out with has absolutely nothing to do with her, Yuna still enjoys helping out here and there. As much as Paine is loathe to deviate from their spherehunting, the whole party knows that Yuna is a total sucker for people in need.