They were from ancient times, so they weren't as open-minded as people in today's society. Women nowadays tried to make their boobs more noticeable and would lack confidence if they didn't have big bosoms. They were even willing to have breast surgery.
In the ancient times, on the other hand, most women felt shy when they had ample bosoms, so they normally wrapped it tightly till it wouldn't attract any attention.
It was the difference in people's values. Therefore, Gu Ning understood why Si Jin did that, but Si Jin had to learn to live in the modern world.
"It's winter now, so we're wearing a lot of clothes, but you still feel embarrassed. In the summer, we normally only put on a piece of thin clothing. Will you lock yourself at home and refuse to go out?" Gu Ning joked.
"I…" Si Jin flushed red. She knew Gu Ning was right, but she was still embarrassed.