A memory, not something most people would consider particularly traumatic or troubling . . . but Snape knew. Snape knew how Harry felt about it, and had lingered on it, drawing it out longer than usual, watching Harry for long minutes as in his mind he sat on his bed at the Dursleys', just staring out the window that summer evening . . . wondering where everyone was. Why Ron and Hermione were together, without him, wherever they were. Why the letters came but no one said anything that mattered or meant anything.
And then the darkness that settled upon him when he found them together at 12 Grimmauld Place. They'd been together, everyone had, while he'd been stuck at Privet Drive, waiting, alone. The total isolation he felt when he realized that all that time they'd been together, Ron and Hermione, without him. If they weren't having fun, at least they weren't alone in Dudley's second bedroom, Cedric's death replaying over and over in their dreams . . .
He brushes the memory away. It isn't fair. It was more than a year ago now, and so much had happened . . . but Snape had made the memory fresh, like it had just happened, just then, that moment. He frowns grimly and speaks the password, stepping through the portrait hole and glancing around the room for his friends.