@Riger Reasons are likely the follwing:
1. It strains her excessively.
2. Enslaving Yuuki is technically her power, so she may feels that only she and her group have the right to use it.
3. Others may try to steal him away from her, such as the commander seems to intend.
4. She believes in Yuuki as a person while the others may view him as a new dog.
5. She likes Yuuki and wants him to herself (the obvious one).