Having decided to get rich quick, thieves found out the best way to do so, is to rob the dragon's cave. They even managed to find friendly wizard, who had potion just for their mission, that would weaken the dragon for couple of hours and stop him from breathing fire for a while as well.
Surely they stole it from spellcaster and spent few days in the forest by the dragon's cave, waiting for the right moment, when it's fed and sleeping.
When the giant have fallen into the day dreaming, sneaky thieves got close to his muzzle and made sure the potion went in his mouth. Just to be safe, they have also tied dragon up and reinforced rope at the top pillar.
But just when everything was done and one of thieves got down from the ceiling onto dragon's paws, to get to the ground faster, he couldn't help himself but lay and hug big dragon foot, which was so attractive and alluring for some reason. His moves on dragon's sensitive paw has awoken the beast, which didn't seem to happy about his situation. But being weakened by potion, he couldn't break his imprisonment nor get rid of it with fire.
That didn't take thief too long to realize, so he decided to take his time and enjoy the feeling, rubbing and scratching around the paw surface, which was kind of tickly for the dragon. All that made him shake and wiggle, starting to laugh and feeling embarassed, feeling so defeated by his ticklishness! Thive's partner didn't take long to get as much as he could and skidaddle, leaving his crazy partner for whatever he had in mind. And it seemed like cave would be filled with laughter for quite some time, before dragon will be able to free himself and punish such impudence.