To answer this classical question, I made a random table - randomness is the answer to any D&D-related question, anyway. It proved to be very practical at first, as it gives both a quick'n'easy background and a personnality trait, without wasting too much time for a potentially dead 1st level PC.
1 – Broken love. You want to forget her/him, or to prove your own value.
2 – Taste for adventure. You just like the thrill of coming death.
3 – Family tradition. Dad was an adventurer. Mum was an adventurer. You got no choice.
4 – Drunkard. You joined adventurers while being drunk. Now you drink to forget you did.
5 – Faith. It’s your religious duty to smite the enemy.
6 – Uniform. These adventurers looks so nice, you wanted to be one.
7 – Family problems. They threw you out and don’t give you money anymore.
8 – Justice. When they caught you, they give you choice: hand-cut, or go far away.
9 – Travel. As a kid, you wanted to see others countries. Now, you do.
10 – Vengeance. Your family was murdered. You’re the arm of vengeance.
11- Poverty. Famine, disease, you need so much money.
12 – Credulity. You really thought it would be easier and you’ll get rich and famous.
13 – Discipline. You were fired off the military. Too much a troublemaker.
14 – Patriotism. Your country is your first love. You will fight for the Land and the Flag.
15 – Feudal obligation. Service of you Lord is a duty and you won’t fail.
16 – Sense of duty. You feel you need to be there, to help people.
17 – Error. It was supposed to be another one. It should have been.
18 – Younger brother. Your older brother will inherit all, money and land.
19 – Sheriff. He wants you, dead or alive. You’re better far away from him.
20 – Glory. You deserve it. Now, you just have to show them.