失去才懂得珍惜错过才知道难得
[shī qù cái dŏng dé zhēn xī cuò guò cái zhī dào nán dé]
This expresses a regretful sentiment, saying that we usually don't appreciate what we had until it's gone or lost, implying an important lesson to learn from past mistakes and experiences with relationships, friendships, objects, etc.