Keeper of the Superfluous Es!

Well, when I did it for Morte O Merce, I fell back on basic psychology: What we hate in other people is what we find loathsome in ourselves. Ergo, the more you hate someone, the more you are really like them where it counts. Factor Snape and Black into that, strip away the things like a wealthy versus a poor upbringing, and the socialization of their school houses, and they really are quite similar. They're both cruel, they're both judgemental, they're both rash when angry, and then guilt ridden when they prove to have been wrong, they're both unforgiving, and they're both in love with their own intelligence.

So, having established what they each hate about each other, to build a relationship between them, all you have to do is find the GOOD points they have in common as well: Loyalty which, once given, is unswerving. Resourcefulness, resiliency, fierce honourability, coupled with a sneakiness which is very useful. Courage, adaptability, intelligence, and magical prowess.

All you have to do then is create a situation where the strengths they share can become more important than the weaknesses. Which is why Hurt/comfort works so well between those two. They will never be romantic, and they will never stop fighting, but that is because neither one of them will ever be really comfortable with their reflection. So as long as you keep the tension between them, it can be really satisfying.

So there: Clear as mud, right?