Even though I love fluff and HEA and romance and PWP and all those good things, the first connection I had to Snarry was through the horrible childhoods they both suffered. I literally cried as I read in the first JKR book about the abuse of the Dursleys and Harry living in a closet under the stairs.
Somehow the damaged Harry and Severus stories seem very in-character, and I particularly enjoy stories that have the two helping each other make a better life together because of their similar childhood abuse. Healing together is a favorite Snarry "magic"!