Bella considers herself to be very close to Tyrone and would often try to be closer to him.

She tries to feel more comfortable around Tyrone by being partially nude around him to make a bigger bond between one another.

(Not just that but since she’s a Furry Anthropomorphic, she just prefers to be nude in general)

Bella doesn’t mind only wearing sweater around Tyrone, but Tyrone feels a bit to nervous when Bella does these kinds of things that it would often make feel more uncomfortable than comfortable.