Why do dogs spin in circles before sitting/ lying down?

Cats often do too. Perhaps it’s more comfortable – i.e. so their fur doesn’t get messed up or pushed into their skin when they sit on it. Maybe so they can find the orientation they like best. Maybe to have a look around so they are oriented to their surroundings from the spot they’re about to stop in, so they can imagine what they’re hearing, remember where they can run at a moment’s notice, etc.?

I think they are matting down the nesting area.

My guess is that it’s because they are about to put themselves in a vulnerable position, so they’re scouting the area to make sure they’re safe. I mean, not really, but sort of as a left over instinct.

I read the circling is from when they were all wild(er) and walked around in circles to mat down the grass to make a nest.

i think they are trying to get comfortable and this is usually associated with a particular smell. this is often why dogs circle before they pee or pace back and forth. they are trying to find the smell that lets them know where to go