My two current characters' motivations are, in a way, quite similar:
Wulgar Rhymesmith, dwarven Cleric/Hierophant of Moradin: His adventuring career started when he (and others) got a plea for help from someone imprisoned somewhere. That someone helped seal away a powerful foe some time ago, and got imprisoned himself.
When we finally freed him, we found out to our chagrin that he was, in fact, that powerful foe. Now, of course, we have to defeat that guy before he can finish what he started last time. In fact, the last session ended with us finally hunting him down and next time, the big final fight will occur.
The other Character, JadeDragon, Samurai (gestalt Kineticist/Warblade) has travelled from Kara-Tur to Faerûn to avert an ancient prophecy about the end of the world, involving defeating an ancient Blood Sorcerer who has now been resurrected and is, apparently, on his way to goodhood, if he's not ascended yet.
I guess we like our campaigns to have an overarching plot.