It is critical to Snape that Harry not find out about his love for Lily. For one thing, we know that’s the reason that Snape’s Worst Memory—the one where he calls Lily a Mudblood—is hidden away from Harry. For another, consider the time Harry uses a Shield Charm and ends up seeing Snape’s memories:
Harry’s mind was teeming with memories that were not his—a hook-nosed man was shouting at a cowering woman, while a small dark-haired boy cried in a corner. . . . A greasy-haired teenager sat alone in a dark bedroom, pointing his wand at the ceiling, shooting down flies. . . . A girl was laughing as a scrawny boy tried to mount a bucking broomstick—
“ENOUGH!” (OP591-592)
One guess as to who that girl laughing was. . . Lily. Snape lets Harry see his abusive home, but as soon as the scene switches to a memory with Lily, Snape pulls away. He does not want Harry to see the friendship between them. Apart from anything else, Snape knows that Harry is the type to throw Lily’s love in his face (Exhibit A: “[My dad] saved your life!” PA285). Snape would never be able to look Harry in the eyes again!
But there’s more to it than that: there is one memory Snape has that would scupper all of Dumbledore’s plans.
“The—the prophecy . . . the prediction . . . Trelawney . . .”
“Ah, yes,” said Dumbledore. “How much did you relay to Lord Voldemort?”
“Everything—everything I heard!” said Snape. “That is why—it is for that reason—he thinks it means Lily Evans!” (DH677)
The entire point of this Occlumency exercise is to avoid Harry being used to get the prophecy. If he hears Snape and Dumbledore discussing a prophecy about his mother, Harry is not the type to put it out of his mind; he would go tearing away to the Department of Mysteries, straight into the Death Eaters’ clutches.
Dumbledore sets up a safety precaution, and loans Snape the Pensieve for the lessons. This is probably not done lightly, given that it contains all of Dumbledore’s own memories, but he trusts Snape not to abuse that. Dumbledore needs to protect the knowledge of the prophecy. And to appease Snape, who is doing a task he really does not want to be doing here, Dumbledore ensures he “shall never reveal the best of [Snape].” (DH679) So Snape ends up with the Pensieve during the Occlumency lessons.
What Dumbledore did not foresee was how the Pensieve actually made it likelier that Harry would see those memories. Snape will take any opportunity to knock Harry down a peg—that’s why he makes such a show of removing his memories into the Pensieve every single time. He knew Harry’s curiosity would be piqued. This is Snape snidely asserting his dominance over Harry, nonverbally gloating that Snape’s memories were safely tucked away whereas all of Harry’s were fair game.
Harry is reckless at the best of times. The combination of Voldemort-related mood swings, constant goading from Umbridge, and Snape going through his memories was a cocktail destined to end in disaster. And so it does when Harry takes the opportunity to dive into Snape’s forbidden memories.
Snape got lucky that Harry only saw the memory he did. But when Snape finds Harry, he cannot be sure which memories Harry has seen. The one he catches Harry in is squarely in the middle, and if Harry saw the other memories in The Prince’s Tale, the results would have been catastrophic.
So when Snape blows up at Harry and brings the Occlumency lessons to an inglorious conclusion, it is not just anger we are witnessing.
It was scary: Snape’s lips were shaking, his face was white, his teeth were bared.
“Amusing man, your father, wasn’t he?” said Snape, shaking Harry so hard that his glasses slipped down his nose.
“I—didn’t—”
Snape threw Harry from him with all his might. Harry fell hard onto the dungeon floor.
“You will not tell anybody what you saw!” Snape bellowed.
“No,” said Harry, getting to his feet as far from Snape as he could. “No, of course I w—”
“Get out, get out, I don’t want to see you in this office ever again!”
And as Harry hurtled toward the door, a jar of dead cockroaches exploded over his head. (OP649-650)
To be sure, Snape is genuinely angry here—Harry just violated his privacy, so Snape bellows and throws dead cockroaches at Harry.30 But while the text says Snape was “white with rage,” his reaction is different from the other times he has a hissy fit (in Prisoner of Azkaban, and during “The Flight of the Prince” in HBP). Here he maintains his sarcasm, and at no point is he described as “deranged,” “mad,” or anything synonymous with that. There’s no CapsLock, no italics, only two measly exclamation points! There’s more going on in Snape’s head at this point than pure fury.
Snape is terrified in this scene, because he realizes how badly he messed up by allowing Harry to get into those memories (and remember, he still doesn’t know how much Harry has seen). The physical description of Snape at that point—lips shaking, face white—is consistent with fear as well as anger. Snape and Dumbledore had just repaired their relationship less than a year ago, and Snape knows Dumbledore would be livid—he would blame Snape for tempting Harry with those memories. When Snape yells at Harry not to tell anyone about what he saw, it’s likely he has Dumbledore in mind, since Snape knows how angry Dumbledore would be after this slipup.
Fortunately for Snape, Dumbledore blames himself (as part of a huge mea culpa at the end of OotP, where Dumbledore shoulders responsibility for almost every bad thing that happened). “I forgot—another old man’s mistake—that some wounds run too deep for the healing. I thought Professor Snape could overcome
