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.
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 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.
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.
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.
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.
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.
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.
Once upon a time, there was a young human girl named Lily. She lived near a mysterious forest. One day, while exploring the forest, she came across a beautiful dragon. At first, she was terrified, but the dragon didn't harm her. Instead, it showed a gentle side. As days passed, Lily and the dragon started to communicate in their own unique way. They shared their joys and sorrows, and gradually, love grew between them. However, their love faced many challenges from the outside world, but they were determined to be together.
One possible fanfic could be that Hiccup's transformation into a Dragon Toothcup is the result of a magical experiment gone wrong. Maybe a wizard in Berk was trying to create a new form of dragon - human bond and accidentally merged Hiccup with a special dragon toothcup. As a Dragon Toothcup, Hiccup could have unique abilities like being able to sense the emotions of dragons more acutely and communicate with them on a deeper level. His friends would be both shocked and intrigued, and they would have to figure out how to reverse the transformation.