Albus Dumbledore is not obsessed with Gellert Grindelwald.
That is rule number one now, and the fine print below it reads Albus Dumbledore is most certainly in love with Gellert Grindelwald.
While Albus doesn't necessarily follow the rule or its footnote, he doesn't break them, either.
"Where's Gellert? You've run him off again, haven't you?"
"No. He ran off on his own. He was mumbling like mad about some papers or books or something."
Albus frowns. If Gellert had only told him -
"You're obsessed, Al."
"What? I am not."
But Aberforth glares at him poisonously, and Albus looks away because he knows, they both know, the truth. When Albus sees Gellert returning through the cool window he goes to the door without looking at his brother, afraid of shame. He can't reach the doorknob soon enough.
Gellert is trailing the books in the air behind him, and one-by-one the glossy, colorful leather-bound tomes follow him in a way that Albus understands - it is so easy to follow the blond. Gellert sees him and Albus, somehow, seems to melt in the hot summer sun and Gellert's warm and thoughtful gaze. Albus has a dim idea of calling Gellert's name out but maybe it's caught in the wind, because Gellert only smiles and smiles at him.
"Obsessed," Albus whispers to himself, "I am obsessed with Gellert Grindelwald."
Rule number two: the rules can change.