It all stems from a paragraph in Deathly Hallows, explaining why Voldemort couldn't get into Harry's head following the death of his favorite former indentured servant. "Just as Voldemort had not been able to possess Harry while Harry was consumed with grief for Sirius, so his thoughts could not penetrate Harry now while he mourned Dobby," the book says. "Grief, it seemed, drove Voldemort out... though Dumbledore, of course, would have said that it was love."
The Reddit poster then points out that both things can be true, since "grief is all the love you want to give but cannot." That brings up an interesting idea. Maybe Snape was safe from Voldemort's brain-jiggery, not because he had the wizarding world's most stoney poker face, but because he was constantly in a state of grief. Snape had been mourning the death of Lily for close to two decades, and the feeling never left him as evidenced by his patronus. Boom, chalk up one more reason that Snape is the Harry Potter universe's most admirable sad boy.
It's a fascinating theory. Does it hold up? Maybe. It's up to J.K. Rowling to decide whether or not it's canon, so we'll have to wait to find out until she's done explaining the important things on Pottermore, like how wizards go to the bathroom.
