Whoops I took a break for a while. Anyway, you always get on a horse from the left side because back in the day a right handed person would have their sword on their left hip. Getting on the horse from the right side would likely result in the horse getting jabbed with the sword. If you’re riding down the right side of the road you’d have to go out onto the road to get on your horse. But on the left you can more easily mount and dismount if you gotta take a piss or whatever.

Also if you’re passing some one on the left, your weapon is in your right hand, so you’re ready to stab if need be. Of course it was customary to raise your right hand to show you had no weapon in it when you met someone, but you gotta be ready to draw and fight. Fucking bandits are everywhere ya know. So keep an eye on the other guy’s right hand as you pass by, just in case he tries something.