在一起不懂的什么是珍惜离开后才知道什么是孤单
[zài yī qĭ bù dŏng de shén me shì zhēn xī lí kāi hòu cái zhī dào shén me shì gū dān]
When together, they do not understand how to cherish each other, but once apart, they realize what it means to be alone. It reflects regret over lost love and missing opportunities.