stolen fruit is (the) sweetest

That which is illicit or illegal is all the more enticing simply by being illicit or illegal. Some people end up cheating on their spouses and partners not out of some deep affection for the other person, but because stolen fruit is sweetest. Why slave away all day to buy nice things with our hard-earned money when there are ways to get them for free? Stolen fruit is the sweetest, after all.

stolen fruit is sweetest

and stolen pleasures are sweetest

Prov. People often enjoy illicit things just because they are illicit. To judge from the number of his extramarital affairs, John must believe that stolen pleasures are sweetest.