"You're not respecting my boundaries!"
No, dude, "I'm gonna leave shit everywhere behind me and you're not allowed to say anything about it" is not a bloody boundary.
Useless phrases from either side:
"You ALWAYS..."
"You NEVER..."
"I've said it 1000 times..."
Sweeping statements like this aren't helpful, ever.
Years ago I read a sort of "template" for constructive arguments and I try to follow it for all potential conflict.
◾️ Be specific about the behaviour that is causing conflict. "You're horrible to me" is too vague. "You spent our joint fun money fund on a ludicrously expensive Xbox game that only you will play, without discussion" is much better.
◾️ Concentrate on the behaviour, not the person. "You haven't done the household chores that are allocated to you" rather than "You're lazy!"
◾️ Describe the practical result of their behaviour. "You didn't take the bins out, which means we have no bin space at all until the next collection in 2 weeks."
◾️ Describe the emotional result of their behaviour. "I feel very taken for granted when we are both working the same hours and have the same leisure time, that you don't complete your chores. It makes me think that you value your time more highly than mine."
◾️ Ask them how they intend to resolve the issue. Be ready to put forward some suggestions that you think would work.
Avoid using words like: Always, never, obviously, clearly, allegedly
Concentrate on the behaviour, not the person
Focus on a solution that meets the needs of both
Do as much listening as talking
Be prepared to compromise
So you might end up with an opening statement like:
"Kevin, I came downstairs this morning and found that you had left all of the lights on and the front door unlocked. That is the third time in a month. Leaving the lights on has cost us electricity for no reason, which means I can't afford to do XYZ unless you're going to make up the shortfall. Leaving the door unlocked means we couldn't claim on the insurance if we were burgled. So if I'd come downstairs to find the TV and laptop missing and neither car was on the drive, we would have no way of replacing them. It makes me feel unsafe in my home and that you don't have any respect for how hard I work to pay the bills. How are you going to ensure this doesn't happen again?"
I have used this approach for years now and the good thing is that it works in pretty much any situation or relationship. I've used it with partners, friends, children (teens and up) and my direct reports at work.
A lot of people say "Take the emotion out of it" but to do that is quite invalidating when you're dealing with behaviour that is painful on an emotional level. How, for instance, can you tell someone to stop swearing at you and make it meaningful without saying how hurtful their words are? Children, in particular, need to understand that their behaviour and words can wound.