For the underworld megadungeon, in case one of the players makes the mistake of asking, "So, why am I here again?"
1 Grandmother is angry at you again. Your mother/spouse/roommates are fed up with all the howling and banging so you have to go figure out what you did this time and fix it.
2 Little brother's cat died. You've decided to find and trap its wandering, tortured spirit for him so he can "learn about life."
3 Accidentally killed your girl(and/or)boyfriend. You feel kind of bad about that. Go find him/her so you can properly lay them to rest, or at least say "sorry."
4 You've grown up all your life with stories about some treasure hidden by one of your ancestors. After a couple years (days) of searching, you've given up finding it on your own, and are determined to find the old coot so he can give you a map.
5 Last week your cousin showed up with some really great pearls that s/he said were from the underworld. Convinced that girl/guy you've been kind of sweet on to go with them to Moon Dance. You're going to go down there and find something better, damn it.
6 You're bored, and otherwise sort of useless.
7 You're pretty distractable, and some joker told you there were shiny things down there.
8 Your older brother stole one of your favorite possessions and says he hid it in one of the odd passageways. You're not sure you believe him.
9 One of your friends died in a freak carp accident and you're actually kind of sad about it. Go find him so you can say goodbye.
10 The cats have been acting really strange lately. Your great-aunt says that means a malevolent spirit of some kind has been bothering the halls of the family dead. You're either the oldest sibling or the stupidest, so it falls on you to go clear it out.
11 You owe money to the mob. This is the fastest way to get it without doing any real work, and even if you don't get it, maybe going down there will convince them that you're too scary to mess with anyway.
12 You want to be a mighty hero, and you're looking for ancient warriors to learn from. It hasn't occurred to you yet that it might be better to look for warriors who haven't gotten themselves killed yet.
13 The local temple needs the poison of a ancestor-touched spiny toad to complete the initiations of their latest batch of acolytes. Somehow you've been drafted into catching a few for them.
14 You want to catch a spirit for a familiar. If you're not a magic-user, you'll just catch a rat or something and call it your familiar.
15 Spirit blossom grows down there. You want some.
16 Someone owes you money and the bastard ran down into the underworld to hide from you. You're tempted to just let him get eaten by spirits, but he's owed you that money for a long time, dang it.
17 You have a question that you desperately want answered, and you haven't been able to find a solution anywhere in the surface world. The last person you talked to said a particular ancestor or spirit might have the answer.
18 Zombie outbreak got the dead all riled up. Take some rice, holy water, and the severed head of the sorcerer responsible down there and calm the lot of them down.
19 You found a skull and you don't know whose it is. Now you need to go find its owner so you can make sure it's not a restless murder victim or something.
20 You're dead, Jim. 50% chance you've noticed.