https://en.wiktionary.org/wiki/forbidden_fruit_is_the_sweetest

forbidden fruit is the sweetest

Etymology

A reference to the story in the Bible in which God tells Adam and Eve not to eat the forbidden fruit, but they are tempted and do so anyway.

Proverb

• Forbidden things seem more appealing; people desire things more if (or, in some way, because) they can’t have them.