Howl was a beautiful person, and he knew it.
He knew it every time he looked in the mirror, or when he stopped on his stroll around Kingsbury to catch his reflection in a shop window.
The truth was, Howl was remarkably beautiful.
There was only one occasion, or instance, where Howl doubted his mirror. There was only one occasion where Howl was afraid to look in the mirror in fear that he wouldn't see the beauty he saw before.
She had done this to him.
She had made him self conscious,
And she would have to pay for this. Of course not physically, She scared him more than any curse from the Witch of the Waste.
Whenever Howl caught sight of Sophie ferociously sweeping the wooden floors of the carpet, or whenever he saw her mutter to her dresses, he was struck with intense beauty.
How could he ever have called himself beautiful when there was this goddess standing in his bedroom. How could he ever call himself beautiful again when she was around. Clearly there was competition, but Howl didn't mind one bit.
Clearly he didn't know true beauty until Sophie forced her way into his life.