What Arborman said. And I don't think it's "psychological", I think it is deepdown hardwired genetics. They have to leave home, right? They have to be able to head out on their own, and if everything in the nest was absolutely wonderful they might look out and see a cruel cold world and never leave. You dont want him there when he's 40! So they get narky over nothing, they get gnarly and unreasonable and convinced they are being most foully mistreated and...
just before you decide that's it, I'll slit his throat when he's asleep tonight they head off to College or get a job and their own apartment and...for ayear or two you'll hardly see them and then bang, everybody is friends again.
But jeebus, from about fifteen to about eighteen or nineteen it is hell. You can't win. SO can't win. Kid can't win. Do you have a brother or are there grandparents nearby where the kid can go for a couple of nights and everybody de-stress?
If this is a "blended" family it's sure to be worse!
I actually "felt sorry" for my ex's girlfriends because my kids were just foul to them. (yeah, even my darling wonderfuls!). When my golden boy was living with me his butt ugly nastiness was focussed mostly on his biological dad and finally I blew, had me a demonstration of Scots fury and said I didn't care how much they hated each other, or why, in MY house they would be polite to each other or I was going to get mad and "take measures".
That was always a de-stressor with my kids. I'd threaten to take measures and they'd start Oh god, no, not the measures, mom, please, no measures and we'd finesse it with jokes...but with golden boy and dad there was no joke possible...the situation was worsened by the fact each of them was "right" in the why of it all and dead wrong in the "how"...
Next time they get going at each other why don't you get up, get the car keys, go out, and drive off for two hours. Not a word from you just.... gone.
They're too big to be sent to their room for a time out so you take your own time out.
And the kid is NOT entirely at fault here. Especially if it's a "blended" situation. Step-parents really need to take a course which will teach them to mind their own fekkin business!!