If you have chosen to be a good guy thief (and these CAN be successful) you may be driven buy rage against unequality in the social structure, perhaps you feel the government is unjust so you steal from it, or even better, you steal for the greater good of everyone (and this does include yourself) but there is a line between greed and doing something for a different reason.

A good resource for thief personalitys is literature as well as the AD&D 2nd Edition Thief's Handbook. Figure out the following and you are on your way to a well thought out thief.

1. Why have you chosen this, often looked down upon profession?
2. What drives you to steal or do what your thief does?
3. Are you a helpful thief or a blight on society thief?
4. How do you react if you are caught, or, do you actually use thieveing skills for the good of the realms?

All these are important starting questions for a thief. You will need to know these answers and more so that you can play your thief to its fullest. Like I have said elsewhere, Thieves are the best roleplaying guild next to bards so, ROLEPLAY!