For one thing, Severus Snape is a bullied geek, and, as Jodel remarks, many of Rowling's adult readers self-identify as geeks or nerds. I dare say quite a few of us were bullied by people like James, Sirius and Lily; as a result, we may well have strong fellow feeling for young Severus when we see him in the same situation.

YES. Exactly. Nerds unite!

So, those of us who, for whatever reason, identify with Severus want to see justice for him. We want him to achieve some peace and happiness, and that never happens in the text. This is frustrating, so we can't let go. We keep struggling to affirm his heroism and discover other possibilities for him.

You've nailed it right there. We've been there ourselves, we empathize, and at least in fiction we can do something to right the wrongs. ;-) Good points.

While I agree that Severus was clearly depressed, I think (as I've commented in some of the comments in the_bitter_word's essay) there's quite a bit of room for speculation and interpretation as to the extent, the duration, and the effects and manifestations of that depression in Severus. Did it completely cripple him for life? Or was it something that was present in a more low-key way later in life, something he struggled against and lived with while never completely healing from it? Etc.