A "souvenir" , from french, recalls a memory. Lethe ( lee-thee with a hard th) means forgetfulness; and nepenthe ( ne PEN thay ) is a drug that induces forgetfulness or oblivion. theres no reason why...
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
Some words do not have direct opposites [antonyms]. Antonyms do not usually substitute for phrases. The phrase "you are" has no antonym. A contrasting version of "you are" is "you are not".