It's complicated, LOL
I guess I'm 'naturally' a side sleeper, but I have really bad reflux, so over the years I started sleeping on my right side. This seemed to be more comfortable, for no other reason than the stomach acid wasn't as bad sleeping on my right side, versus my left. I'm guessing it has something to do with how well my stomach 'retains' stomach acid, depending on the side I sleep on? That's probably a question for Dr.
@Izo
But sleeping on my right side isn't a 100% fix. I will still get reflux in my sleep when sleeping on my right side. Even better is building up a huge pile of pillows and trying to sleep on an incline, flat on my back. And this is probably the best way for me to do it. But what ends up happening is that once I fall asleep on my back, inclined on pillows, is that I will flop over to my right-side again, except now there's a giant pile of pillows under my head, and my neck ends up at a 45 degree angle, or worse, to the point that my left ear touches my left shoulder. Somehow this doesn't bother me while sleeping, but after I wake up I will have a horrible pain from my right trapezoid, up the back of my neck, and sometimes the pain will continue on up and over the right side of my skull, almost to my right temple.
Shit fucking sucks, man