Because you have a power that Aizen enjoys. After you are taken by Ulquiorra to Hueco Mundo and you are taken Las Noches Tower under Aizen's orders.

You've changed into your Arrancar uniform and are in a room with only a slight glow of the moon. You can't help but worry when you're in this place, no matter how strong you are. Ulquiorra opened the door of his room and entered your room, his face still expressionless.

"By Aizen's orders, I will be your supervisor, girl." His eyes look at you with a cold expression as he speaks in that deep tone.