You shouldn't let people tell you how to eat.
John's friend tells John not to eat the french fries, as they are fattening.
John doesn't want to offend his friend.
John tells his friend that it's his cheat day for his diet and it's a one-time treat.
John agrees with his friend and doesn't eat the french fries.
John's friend agrees and leaves John alone.
John wastes his money that he spent on the fries and goes hungry.