What are your Snirius Headcanons?
Some general things, although for me this kind of stuff always depends on the story you want to tell. Ship dynamics always depend on context for me, but speaking very broadly:
Doomed from minute one. Nothing good is going to come out of this, not for them and not for the rest of humanity either.
Except for the sex. The sex is probably amazing. Mostly because the more they argue, the more they sleep together.
As a general rule, they almost never agree on anything, and that's for the best, because if they ever do agree on something and that thing requires them to take action, God help us all.
They constantly bring out the worst in each other, and weirdly enough that's exactly what attracts them. It's not an "I want to fix him" dynamic, it's an "I want to make him worse" dynamic. In a way, they like not having to perform goodness around each other. They both have very dark sides that they usually have to mask in order to seem socially acceptable, but around each other they don't need to because they know they're both terrible people, so it's actually kind of liberating.
Sirius is very much the type to let everything out, while Severus keeps everything bottled up, so there's a huge clash of misunderstanding there. Especially socially, because Sirius is good at parties and people, and Severus absolutely hates all of that.
Sirius drinks more; Severus handles alcohol better.
They both have awful senses of humour, and if they ever stop using it against each other and choose a shared victim instead, that person will probably end up in therapy that very afternoon. You do not want those two teaming up and saying horrible things because together they're more toxic than Chernobyl.
They physically fight more than once. Sometimes they have sex afterwards, sometimes they have a drink instead. Depends on the day and the mood.
They're the kind of couple who mentally exhausts everyone around them because everything always ends in drama. The sort of people where if they show up together at a gathering everyone is quietly praying they don't start something today, because if they do, they somehow turn their problem into the problem of every living being present. They completely drain everyone's energy.
Divorced energy even when they aren't in crisis. Their stable periods basically feel like exes who get along and sleep together, but still: divorced energy.
They're both unbelievably stubborn and absolutely incapable of backing down, and both always have to get their own way, each using different methods. So living together is basically a battlefield.
Fascination and an incredibly codependent attachment to that battlefield.
They're both comfortable operating within conflict because they basically grew up in environments full of conflict, so that's what they learned. Sirius doesn't know how to relate without imposing himself and shouting; Severus doesn't know how to relate without being defensive. Pure Molotov cocktail.
Both of them are girl-dad coded, but in completely opposite ways.
You can always tell from their faces when they can't stand someone or don't want to be somewhere, and neither of them ever hides it. So if they both dislike the same person, they'll spend the entire evening staring at them with complete disgust.
If they live together, their house is an absolute disaster. You'd walk in and immediately want to die because of how messy it is. Sirius is a posh guy who doesn't know how to fold a shirt, and Severus is depressed and sometimes completely gives up on everything. Absolutely terrible.