What motivations are there for megadungeoning that matter to the PCs? Is gold, glory, and exploring the unknown enough, or does it need to be something personal?The answer, of course, is that it doesn't need to be something personal. Actually, the explanation/motivation really ought to be, at the most basic level, "You're playing a game, the DM has gone to the trouble of creating a cool megadungeon, so go and explore it, for goodness' sake."
That said, I'm enough of a 1990s gamer to think that having pseudo-emotional connections amongst PCs, and between PCs and NPCs, lends weight to what is going on at the table. So why does it always have to be gold, glory and explore the unknown? Why can't there be a bigger reason than that?
Reasons For Entering the Dungeon, Part 3
1. To rescue a missing son, daughter, wife, husband, cousin, niece or nephew kidnapped by slavers or worse
2. To recover the remains of a slain friend so he can receive a proper burial
3. To find a family heirloom which your adventuring father lost as a young man while fleeing the terrors within
4. To impress a potential bride or groom
5. To gather enough treasure to pay off crippling debts
6. To conquer inveterate cowardice
7. To find a cure for a sickness affecting a beloved family member
8. To find a cure for a sickness affecting yourself
9. To wreak terrible revenge on a hated foe who has hidden in the dungeon's depths
10. To search for the rumoured El Dorado-style utopia which lies in the roots of the world