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Let us just say - I was deeply unhappy, but I didn't know it, because I was so happy all the time.
— Harris · L.A. Story

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A paradox about not noticing your own unhappiness because you were constantly cheerful. The speaker suggests that relentless happiness can act as a cover, hiding a deeper dissatisfaction until later, with the phrase "let us just say" giving it a rueful tone.
Context reviewed