Like anything starts. Ideological differences.

[ Severus doesn't really want to talk about the war. He had to give context, though, for the actual thing they're talking about - that he still would rather not, but it's worth it to make sure someone else knows, perhaps. ]

The perception of ideological differences is one of the reasons why Black hates me, despite the fact that he's had the concept of 'covert agent' explained to him dozens of times. [ So ... there's that. An indirect admission but an admission all the same. He'd honestly prefer that no one know, but someone had the bright idea to tell Sirius before he came here, and Sirius has blabbed about it extensively. His only small comfort about it is the idea that should the Dark Lord arrive, Black will be one of the first to go. ] I think he clings to it. Otherwise he'd have to admit that he hated me first for having the audacity to be poor and ugly while getting better marks than him in school. 'Bully' is an understatement. He tried to have me killed when we were sixteen. Nearly succeeded.

[ His cold, clipped insistence of origin only might make more sense, now. ]