Perhaps it's a form of protection. The dragon might sense some impending danger to the girl in her human form. In the dragon world, she could be stronger and more able to defend herself. Dragons are often seen as powerful creatures, so the dragon might think that by turning her into one of its kind, she will be safer from threats that a human could not easily face.
One possible motive could be loneliness. Maybe the dragon has been alone for a very long time and wants a companion that is more like itself. By turning the human girl into a dragon, it creates a being that can understand its dragon - like experiences, such as flying, breathing fire (if it can), and having a connection to the dragon world.
The last dragon, an ancient and wise creature, noticed a human girl who was always kind to the animals in the forest. The dragon knew that the world was changing and that dragons might disappear forever. It decided that the girl could be a new beginning for dragons. Using its powerful magic, it transformed the girl. As a dragon, she was confused at first. But the last dragon showed her how to hunt, how to find a safe place to sleep, and how to communicate with other magical creatures. Over time, she embraced her new identity and became a symbol of hope for the future of dragonkind.
One possible reason could be that the female dragon sees something in the human girl that she wants to preserve in a dragon form. Maybe the girl has a special quality like great courage or a pure heart that the dragon thinks would be enhanced as a dragon. Another reason might be that the dragon is lonely and wants a companion of her own kind, so she turns the girl into a dragon.
One reason could be to preserve the dragon lineage. Since it's the last dragon, it might see the human girl as a way to continue the existence of dragons.
At first, she might be in shock and confusion. But as time passes, she starts to discover the physical abilities of her new form. For example, she learns how to use her wings to fly, which is a completely new experience for her. She also has to get used to her enhanced strength and senses.
The plot could start with the human girl being in a dangerous situation like lost in the dragon's territory. Then, the dragon, instead of seeing her as a threat, decides to protect her. As they interact, they learn from each other. For example, the girl could teach the dragon about human emotions like love and friendship, and the dragon could teach the girl about survival in the wild.
Maybe she uses a magical spell. Dragons are often associated with powerful magic in stories. She could chant some ancient incantation and wave her dragon claws in a specific pattern to cast the spell on the girl.
Perhaps in the fanfic, Naruto turns into a dragon as a result of a forbidden jutsu gone wrong. In his dragon form, he discovers a connection to an ancient dragon spirit that guides him. This spirit could teach him new abilities and knowledge. His friends, like Sasuke and Sakura, would then have to help him figure out how to reverse the transformation or at least learn to live with it. They could go on a journey to find a way to control the dragon within Naruto, facing various challenges along the way, such as other ninja clans who see Naruto's new form as a threat.
One possible theme is acceptance. Since dragons and humans are so different, they have to accept each other's natures. For example, a human might have to accept the dragon's wild and powerful side, while the dragon has to accept the human's fragility. Another theme could be sacrifice. Maybe the dragon has to sacrifice its freedom to be with the human in the human world, or the human has to sacrifice their social standing to be with the dragon. Also, the theme of transformation can be present. The dragon or the human might transform in some way because of their love, either in personality or in how they view the world.
A human girl, Emma, lived in a small town at the foot of a great mountain. A dragon had made its lair on that mountain. One day, the dragon, a huge and powerful beast named Thorn, flew over the town. Emma was not afraid. She stared at Thorn with wonder. Thorn noticed her courage. He landed near her. At first, Emma was a bit scared but then she saw the intelligence in Thorn's eyes. They formed an unlikely bond. Emma taught Thorn about human ways, and Thorn showed Emma the beauty of the skies as he took her on short flights.
Another possibility is that the girl was being punished. In some fictional settings, dragons are seen as guardians of certain places or secrets. If the girl violated some unspoken rule or tried to steal something from the dragon's domain, the dragon might eat her as a form of punishment.