But he was a young dragon, and he was not allowed to leave the nest, and so he sat, and brooded, and aged under the weight of his brooding as his lithe form filled out so that his tail could wrap 'round his cave three times and, finally, he could go.
The first thing first was to kidnap her. He could gather any amount of gold in the world and while it would be a good start on a hoard, it would not be of value to the world until he had a princess in a gilded cage. And so he slid out into the night, letting his wings catch the air, and felt alive for the first time since his birth.
He found her easily enough, of course; a dragon always can find a princess, especially one who he wishes to hoard. And so he slid into her room, scooped her up in his claws, and carried her off. She did not scream, or cry, or do any of those things that might have shown she'd have forgotten him; instead, she wrapped her arms around him so that her silk skin and her silk gown pressed against the slide of his scales, and she pressed her cheek into his neck until the sharp edges almost cut her, and she whispered, "You came back for me."
He knew then that he'd love her forever. And of course, it is not fit for a dragon to love a princess -- to keep her, yes, to hoard her, to make her his best possession, oh yes. But it is against the rules for them to love, and it didn't matter. He brought her back and laid her down and wound his body over hers, and he thought that she must be dimly aware that she should be afraid, embarrassed, shamed -- but she was not, arching under him, wrapping her arms tighter, her legs opened wide around his bulk, a whimper rising in her throat as he slid against her, careful not to hurt too badly.
And so the dragon took the princess, and so the dragon loved the princess, and she called him beautiful -- more things that a princess should not call a dragon -- and rubbed the ridges of his wings and dreamed of an eternity of capture.
But in worlds such as these there are princes who will come after a princess for no other reason than that they want a princess, and while the dragon was out one day to gather food, a prince's retinue entered and stole her away. This time she screamed and cried and begged them not to take her, but a prince will have a princess and the dragon had let the wind carry him too far for him to hear anything.
When he returned and found the cave empty, he was filled with a fury of the sort to do proud to all the most ancient dragons, and he rampaged on the nearest town, the town he'd originally stolen her from, burning the homes, tearing down the castle, until there was nothing of it left from the richest noble to the poorest orphan. And then on, tracking her scent and those of her captors, until he found the castle of the king and prince who had taken her, and their retinue of a thousand.
The dragon rampaged there as well, furious and proud, ignoring the arrows that pierced his scales, ignoring the cries of the helpless servants as he tore them to pieces, ignoring everything but the vague sense of her somewhere in there. He slid his long scaled body through corridors, down stairs, devouring and mauling as he went until there was nobody left but a princess in a cage.
But a princess is only a princess so long as she is pure and clean and an object to be worshipped. She is still a woman afterwards, of course, but the princess wasn't content to be a woman, nor a mother, nor that she had had a man inside her rather than a dragon, nor any of a dozen other things, and so when the dragon reached his talons towards her to pick her carefully up and bring her carefully home, she drove herself on them as well.
He wailed, as only dragons can wail, setting fire to the castle with his breath, and so stricken was he that he barely noticed the prince walking towards him.
The prince, it turned out, was an enchanter, and he was none too pleased that his princess had been taken from him so quickly. Princes do not need to care about their people or their kingdom -- that is a king's job -- but a prince requires a princess, after all; the hunting of one, the marriage of one, the keeping of one for the future or throwing her away to acquire another. And so, for his crime, he put a curse on the dragon that he must become a man, and walk in a man's skin, and live as a man -- weaker than a dragon in some ways, stronger in others, but the shape was the thing; it was men who had taken the dragon's princess, after all, and men whose touch she could not bear.
The dragon-who-was-a-man killed the prince, but it did not turn him back into a dragon, and he thought it was just as well; as a dragon, he would need a princess to hoard, and he would never find another. As a man, he had nothing, needed nothing: It was a world of nothing for him.
Of course, men who have nothing are beggars, and so, broken and bleeding and dying from the many injuries gained when he had attempted to rescue the princess, he took on his role as a beggar and went out to do what beggars do: Receive alms or die. It is there where the dragon's story ends and the man's begins, or shortly after.
(He does indeed find alms, and charity, and through those could change himself to something else, though never a dragon again -- but that is a different story, and must be saved for a different time.)