The staffroom, a long, paneled room full of old, mismatched chairs, was empty except for one teacher. Professor Snape was sitting in a low armchair, and he looked around as the class filed in. His eyes were glittering and there was a nasty sneer playing around his mouth.

As Professor Lupin came in and made to close the door behind him, Snape said, "Leave it open, Lupin. I'd rather not witness this." He got to his feet and strode past the class, his black robes billowing behind him.

-Harry Potter and the Prisoner of Azkaban, Chapter 7: The Boggart in the Wardrobe

Professor Severus Snape's boggart had changed quite a few times in his life. Most people would have a fear such as spiders, clowns, or heights, and that would never change. Professor Snape had experienced many horrible things, however, and he was not "most people" by any means.

When he was growing up, Severus's greatest fear had been his mother dying. Tobias Snape, his father, was a very cold and unloving man, who shouted a lot and would leave the house unannounced for many days at a time. He was an all around hateful man, and if his mother died Severus wouldn't know what he would do.

But then, she did.

And his fear changed.

Severus wasn't exactly sure what his greatest fear was now, and he didn't find out again until his third year. He had found a boggart in his wardrobe after waking up late, and had barely been able to fight it off before collapsing onto his bed from grief. It had been Lily leaving him, telling him that he was horrible and that she hated him. Lily was his only friend, the only one he'd ever had. He'd never want her to leave him, because then he'd be completely alone.

Then one day he spoke out of anger and humiliation, and she did.

And his fear changed.

It didn't matter to Severus that Lily had left him, Snape would always care for her. It was a horrible mistake to tell the Dark Lord of the prophecy, but how was he suppose to know that it could possibly refer to her and her family? He knew now what his greatest fear now would be. Lily, dying by the Dark Lord's hand. It was all the worse knowing that it'd be his fault. It was a little comfort to know that she was under the Fidelius Charm. The Dark Lord had no possible way of reaching her now, did he?

But then, the Potter's were betrayed, and she did.

And his fear changed.

Snape would never face his boggart again, not if he could help it. He already knew what it was, and would never admit it to another soul.

Harry Potter, motionless and cold, his bright green eyes glassy and unblinking. His last link to Lily, dead, murdered by some scheme of Voldemort, or a Death Eater seeking revenge, or something else along those lines.

And Severus Snape knew that he'd do all in his power to never let his fear change.