My last character, who recently died at the hands of a half-stone golem/half-minotaur, adventured to demonstrate the superiority of the elven longbow over the forces of darkness. And also to protect his wife.
My current character, a Rhennee rogue/fighter, has joined the adventuring party because:
1. The party saved him from the Deepmother, and he owes them.
2. He has no other sort of family being an outcast among his own people as well as polite society.
3. He's wanted by the authorities and the thieves' guild in nearby Verbobonc, and the extra adventurers could help cover his hindquarters.