...ok, so flipping the y-axis.
I reckon that I do it because my first 3D game was Microsoft Flight Simulator (I haven’t looked this up but I’m pretty sure up and down are flipped in that game by default, like a real plane).
My brothers, meanwhile, who are all younger than me and had their first 3D experience on PS2/etc all prefer standard y-axis.
So what got you into flipped y-axis, if you use it? Something in real life that you were used to before getting into video games? One specific game that locked you into it?
I used inverted mouselook right from the beginning of mouselook in FPS games up until 2006 or so. I have no idea how or why I switched, but there was a weird period where I couldn't remember which one I preferred and both felt wrong. There was an even weirder period where I liked inverted for general aiming but non-inverted for sniper sights.