One of my players burst out with a gem at our most recent game session.
The players started to talk about staying in the dungeon, protecting it from other adventurers, and otherwise ensuring they maximize their loot without letting anyone else get any.
"Is this how dungeon monsters get started?"
Step 1 - find the dungeon.
Step 2 - start to exploit its treasures.
Step 3 - worried about rivals, you start to camp out at the dungeon, set traps, ally with monsters in the dungeon to help you protect it, etc.
Step 4 - you finally move into the dungeon full time. You can only protect so much, so you start locking doors, setting traps, blocking off passages, and creating your own little patrol area/safe zone. You stay there with your magic items and treasure.
Step 5 - Other adventurers finally show up, and you instantly and suspiciously attack them. They're clearly the other adventurers come for your stuff that you've feared this whole time. They're after your stuff and that of the other monsters whose loot you covet for yourself.
Final stage - Before you know it, you went from "Borriz Borrizman, bold dwarven delver!" to "that crazy dwarf in Room 23."
So if you ever wondered about the Lich-King down on level 11, well, he started as an adventurer and just didn't leave. Now he's down there, and yes, he does regard all of the treasure in the whole damn dungeon as his. That's why he moved into the dungeon into the first place, to protect it from you!
Thanks to Aaron, Borriz's player, for this eminently logical train of thought.