She is too timid and worries too much. She didn't dare to try a relationship when she shouldn't, and would rather keep it carefully in her heart and never make it public for the rest of her life. She knew clearly that a broken mirror would be difficult to put back together, and even if it was glued back together, it would no longer be the complete mirror. So she didn't dare to go any further, and she refused him to go any further. She was afraid of being separated, and she was also afraid that the person she loved would disappear into the sea of people. She threw all her worries to tomorrow, just because tomorrow was a day that would never come. Even at the final moment of separation, she only made a "promise for tomorrow". There are so many variables in the future that all she needs is a promise, whether it comes true or not.