People have always forgotten that he used to be a teenager with a black beard, who was unscrupulous in idealism and love.
Dumbledore has many secrets. He loves that blond teenager, and he broke the blood alliance. He used to have the same ambition and idealism. If he wanted to reconstruct and conquer the world, he killed his former self. He must admit that he can't blend in with other people's happiness outside that bakery, but the person he misses is defeated by him; He can't deny that he has always loved Grindelwald and has never changed.
all this is his secret, but it will be preserved in the end, and he will never see the light of day.
Since then, "You are alone" has hurt both of them.