1. You're on the run from the law, and you figure the town watch isn't going to follow you into a dungeon just to arrest you for shoplifting.
2. You're desperate to strike it rich to save your family from financial ruin and prove you're not the ne'er-do-well they say you are.
3. Your beloved aunt or uncle disappeared into a nearby dungeon and you want to find him or her.
4. You helped a crazy old galoot across the street and he gave you a treasure map to this place in return.
5. You believe you're the rightful heir to a fortune said to be hidden in the dungeon and you have an heirloom to prove it.
6. You believe a fugitive responsible for the death of a family member is hiding in the dungeon.
7. You're working for a wizard who is paying you to recover a lost artifact.
8. Buy me a drink and I'll follow you anywhere.
9. Your best friend here [pick another motive from this list] and if that's what he wants to do, that's good enough for me, too. We're a team.
10. See this scar? I'm going to get the villain who gave that to me when I was a lad.
11. Exterminate the brutes. Every last one of them.
12. If I bring back the trinket for them, they'll let my family go free.