Many modern times have written dragons as victims of a greedy and vain humanity, innocent and nature loving and reasonable. These dragons in this story are nothing of the sort.
They consider live virgins to be fine dining, could survive off rocks if need be, but MUCH MUCH prefer eating something with a still beating heart as a matter of principle. They burn villages to the ground and shift through the remains for any precious metals and gems and take it for themselves. They are egotistical to an extreme. Their greed makes the even most miserly human seem generous. Their fierce individuality means they're just as likely (if not more so) to attack each other as a mutual enemy in the heat of battle over loot and kills (or just a chance to dispose of a rival in a distracted moment).
These dragons are PROUD of what they are. They see humans as useful only in the jewelry and treasures they make (which dragons lack the delicateness to make for themselves... or much of anything), and as foot, pets, toys, or all three in the case of young ones.
The youngest are deadly weapons, the adults are armies unto themselves, and the truly ancient are truly forces of nature on par of a meteorite hit.
Then humans had to go and invite such annoying things such as ICBM missiles, atomic bombs, and the like. But only for a time. THEN the dragons decided they wanted these for themselves too! (One dragon boasted having one of every CD and record EVERY MADE in their horde).
So many more dragons took to assuming human form, or at least using human aliases, their intellect and cunning masking the savagery and blood-lust that burned in their veins. In spite of these, dragons still lacked in these forms things like human team work, and human imagination. So the dragons would take those for themselves too. As a species if not as individuals.
So decided Tiamat and Bahamut, the dragon equals to Adam and Eve, both having worshiped as gods by humans before.