Harry doesn't know why.
He can't explain why he feels utter disgust at every petty thief and every would-be-Death-Eater they come across. He sits and works on paperwork that isn't worth it and makes his hand remember another quill and a different punishment. He comes home and casts his robes on the floor and showers until his skin is glaring red and sore and itches for days before forming into scabs. He curls around Hermione in their bed which feels too empty and too intimate without Ron there and he cannot tell her what's wrong but she sense it in his shoulders and the way his hands stutter over her skin and the way he sleeps clinging to her desperately.
He tells Ron first. He knows very well they both consider Ron their best friend, and ached for his happiness so badly that they said nothing when he found Susan. Hermione will ask "why" and expect an explanation. Ron just listens, buys another drink for the both of them, and tells Harry to do whatever will make him happy (he's grown up, it seems). Later, when both boys are sober and Harry braver in his convictions, they tell Hermione. She doesn't understand at first, but she hesitates in her convictions (they've all grown up, perhaps) and listens to them.
She looks at him like she looked at Ron when he left and her hands tighten around her wand. She doesn't say the protests on the edge of her tongue. She forced down the feeling of failure in her throat and suppresses the fury in her belly and hugs Harry so hard he knows it anyway. Ron stays over that night.
Ron tells Susan later (he deserves to be happy) and Hermione and Harry tell Sirius (Lily will be so proud). They spend the rest of the night patching Sirius up and Hermione cancels yet another date and Harry sits and stares at the empty owl perch that has been in the living room since they moved in. He hopes his parents would have been proud, and that Sirius is right.
The news does not spread quickly. Harry goes to work and comes home, waiting for the right time. They tell Ginny one night over fast food to celebrate her team's latest win (you've won the War already, Harry, you don't need to fight anymore) and he breaks down.