Maybe the magician was just bored. He had been practicing magic all day and wanted to do something different. Changing a mouse into a girl was a rather unique and interesting experiment for him.
It could be that the magician saw potential in the mouse. He might have thought that as a girl, the former mouse could have a more meaningful existence. Perhaps he thought she could bring some new energy or perspective to his world. For example, she could help him with his daily tasks or be a companion in his rather lonely life as a magician.
The magician might have been testing a new spell. He had created a spell to transform living creatures, and the mouse was just an unlucky (or lucky, depending on how you look at it) target. He wanted to see if his magic could really change one form of life into another completely different form. And when he saw the mouse, he thought it was a good opportunity to try out his new spell.
Chapter 737, "Shu Yan in Trouble!", is the clearest early answer because it turns Sister Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on South City Film and Television Shooting Base.
Chapter 703, "Slap Oneself", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event.
Chapter 768, "Sorry Everyone", is the clearest early answer because it turns Jiang Zhao Yuan's situation into a concrete plot event.
Chapter 837, "Why did she come?!", is the clearest early answer because it turns Gu Qiang's situation into a concrete plot event. It opens with The ring is the one Shi Lei just gave her, she really likes it, so... and closes on Thank you, Old Madam Gu." Then the Old Madam continued, "I know....
Chapter 885, "Isn't This Wine?", is the clearest early answer because it turns Shuiyun's situation into a concrete plot event. It opens with Daoist Shuiyun's words weighed heavily on Jiang Li's heart like a... and closes on Even replacing all the tea in his room with wine..
Chapter 597, "Uncontrollable Things Still Happened", is the clearest early answer because it turns Dragon Ridge Island's situation into a concrete plot event. Early on, Gu Yu's disappearance couldn't be related to the secret of this.... By the end, Thank God the drone exploded over the lake..
Chapter 652, "He's Gu Shengrong's Grandson?!", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event.
The best chapter pointer here is Chapter 13, "Killing the Chicken to Scare the Monkey". It opens with Sensing that something was off with Jiang Yan, Jiang Yan dismissed the... and closes on How could this happen..." Lin Manru didn't want to hear any more of her....
Chapter 702, "This is One Hundred Million, Sign Here!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 555, "Peaceful Coexistence", is the clearest early answer because it turns Chen Manman's situation into a concrete plot event.