Stealing from a Dragon is a Bad Idea

1. They breathe fire. Are you fire-proof? No? Then stay away from that cave.
2. The can fly. Climbing to get away won't work. In fact, you'll be quite easy to pluck off of a cliff from the air.
3. They have sharp teeth. The better to rip and tear tasty trespassers.
4. They also have sharp claws. Do you really need both arms?
5. They are very protective of what belongs to them. All that gold is the dragon's, and so is that pile of treasure, and that pile of armor and weaponry that used to belong to various knights and warriors.
6. A broody dragon is doubly dangerous. If you see even a hint of a dragon egg, it is too late for you to escape. The mother knows you're there, and she will ear anything to protect her clutch.
7. Dragons are smart. The intelligence of a dragon ranges from "bestial obsession to destroy all enemies", to "having calculated fifty-six ways to kill you and add to my hoard at the same time".
8. Most of them are bigger than you. And the little ones make up in speed and devious cunning what they lack in size and brute strength.
9. Did I mention they have excellent senses? And should one sense be weak, the others make up for it in their sensitivity.
10. Dragons hold grudges. So if you manage to make it out alive, the dragon you just stole from will make every moment from then on a nightmare for you; until it finally robs you of all that you hold dear, destroys any who try to help you, and then only after your transformation into a shattered version of yourself is complete, will the dragon finally kill you in the most painful way it can imagine.