forbidden fruit is the sweetest

• the forbidden fruit tastes the sweetest

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 by a serpent and do so anyway.

• (Received Pronunciation) IPA^(key): /fə.ˈbɪ.dn̩ fɹuːt ɪz ðə ˈswiːtəst/
• (General American) IPA^(key): /fɚ.ˈbɪ.dn̩ fɹut ɪz ðə ˈswiːtəst/

forbidden fruit is the sweetest

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

• the grass is always greener
• the grass is always greener on the other side