No partner, I sleep on the left because that’s where the nightstand and the alarm clock are. The dresser is on the other side of the bed and I don’t want to try to crawl up the dresser in my sleep to turn off the clock. (I’ve been known to try to unlock my phone in hopes of turning off the alarm. Didn’t work, obviously, and nearly locked it for good into the bargain)
Besides, the cats take up the free side of the bed. 
My husband is adamant that he must sleep on the side nearest the door, so if anyone comes in, they have to get past him to get to me. Even when I was pregnant, and had to pee every 20 minutes, I had the side away from the door and the bathroom.
I recently rearranged our bedroom, which meant we had to switch sides, so he can still be nearest the door.
He does the same in hotel rooms; he must be closer to the door than me. I wonder if he’s planning an escape…
She sleeps on the right-hand side. This only incidentally closest to the bathroom - my side is the wall side, and I get into bed by crawling up from the foot like the dog that I am.
I sleep on the right, my wife sleeps on the left. We’ve done it that way for almost 20 years, in different houses and apartments with different bedroom door and bathroom locations. Don’t know why.
I guess I always sleep on the side closest to the alarm clock at my place (which is on the right), and the side opposite of the clock if I’m at a girl’s place. Usually I sleep alone though but I still sleep all the way to the right.
I am male, and I sleep on the left side of the bed. I thought through this years ago and made my decision carefully. I’ve stuck with it for every SO I’ve had.
I am right handed. I almost always sleep with my hand/arm under my SO’s head/neck. Usually spooning. So this’ll incapacitate my left (non-dominant) arm, leaving my dominant hand free to… um… do things. Mostly playtime with SO, but also practical things like retrieve glasses and operate alarm clock.
The way I see it, the playtime concerns far outweigh the door/bathroom proximity concerns:[ul][] So what if you’re on the far side from the bathroom? It’s three extra steps. []Chainsaw-wielding maniac bursting through door? We’re both goners anyway. Baby crying? Wait… WHAT BABY???[/ul]
OK, I’m going to have to weird you all out here. I sleep on either the left or the right side of the bed, as other overriding circumstances dictate.
What overriding circumstances, you ask? Well, invariably, whatever bed we happen to be in, Mr Aspy ends up on the south side, and I end up on the north.
That’s weird, but that’s not what’s really weird. What’s really weird is that (a) it’s generally me who “picks sides” in a new bed and (b) I generally have no idea at all which way is north in a strange place, nor do I think about it much (in fact, in drafting this I got the two directions mixed up and had to be corrected by my husband. Who’s the one who noticed this in the first place)
Mysterious Forces conspire to keep us from ever sleeping in a north- or south-pointing bed. Except just one time. Then I slept on the west side.
I know why this is.
Clearly, wherever you sleep you are subconsciously drawn to the warmest and sunniest side of the bed. If you were lying in a northern hemisphere bed I guess you would opt for the south side, where you would be that much closer to the equator.
I must confess this is a factor I had not previously considered.