曾经沧海难为水，除却巫山不是云
céng​jīng​cāng​hǎi​nán​wéi​shuǐ, chú​què Wū Shān​bù​shì​yún

high standards

曾经沧海难为水，除却巫山不是云 (曾經滄海難為水，除卻巫山不是雲) céng​jīng​cāng​hǎi​nán​wéi​shuǐ, chú​què Wū Shān​bù​shì​yún
high standards
one who has seen the world or reached a high level of expertise is no longer impressed by trivialities or mediocre things
incomparable
once one has experienced the greatest or most beautiful things, nothing else can compare or measure up (often used to describe devotion to a lost love)

Sources
Definitions
CC-CEDICT