Perhaps the dragon is following some sort of ancient magic rule or prophecy. In some dragon lore, there are prophecies that involve transforming certain humans into dragons for a greater purpose. The female dragon might believe that the human girl is the one mentioned in such a prophecy and thus turns her into a dragon to fulfill that destiny.
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.
It could also be a form of punishment or a test. If the girl has somehow offended the dragon or the dragon's territory, turning her into a dragon could be a way to teach her a lesson. Or it might be a test to see if the girl can adapt to the dragon's world and overcome the challenges that come with being a dragon, and if she passes, she may be turned back into a human.
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.
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.
It could be due to a magical artifact. For example, if she touches an ancient and cursed dragon scale or a powerful enchanted jewel. These items often have powers that are not fully understood, and their magic can cause unexpected transformations. Also, it might be a self - sacrifice. In some stories, to save her loved ones from a dragon - related threat, the girl might choose to become a dragon herself to fight on equal terms.
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.
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.
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.
One possible plot could be that Issei discovers a hidden power within himself that gradually transforms him into a dragon. This new form gives him enhanced strength and abilities, but also attracts the attention of powerful enemies who want to either control him or eliminate him. He has to learn to master his dragon form while also protecting his friends and loved ones.
In the fanfic, Astrid could be part of a scientific experiment gone wrong. The experiment was meant to study dragon DNA, but something malfunctioned and she starts to transform. She then embarks on a journey to find a cure. Along the way, she discovers that being a dragon gives her a unique perspective on the world and she has to decide if she really wants to go back to being fully human.