I don't really understand why people keep asking this question. Clearly, she has been with Vaan since they were children-- they're pratically brother and sister. She went along because she wanted to keep Vaan safe.

Quote Originally Posted by Cap'n Basch
I guess people would be happier if there was some empty and ultimately meaningless motivation for them all to be there. "A Princess needs her bodyguards" in the manner of "A Summoner needs her Guardians" or something. But it's not because the characters are somehow bereft of motive that people have this problem - it's that they expect the spoonfed "we're all here for X", whereas the game places a greater onus on the player to pay attention to the characters and work this stuff out themselves.
Exactly. What I like about this game is that all the characters have different reasons for being on the quest-- Basch wants to protect Ashe, Ashe wants to restore her homeland, Vaan wants to avenge his brother, Penelo wants to make sure Vaan doesn't get himself killed, and Fran and Balthier want to stop Cid and scoop up some loot along the way.