I mentioned this in the left or right thread I think, but I have that as well. Bicycle or motorbike, I can only coordinate getting on from the left hand side. The second I try and mount it from the right hand side my brain goes in to some overthinking meltdown and I forget how all of my joints work. From that point on all bets are off, I'll either end up clumsily draped across the bike with a befuddled look on my face as to how it happened, or sprawled in a heap of limbs and frame on the floor begging for mercy. Knowing this, I will do anything I can to mount on the left side, although for some reason once in a blue moon I complete forget this and try to get on from the wrong side, realising my mistake at some point after my left leg has raised beyond 45 degrees, if I'm sensible at that point I'll give up and start again from the 'correct' side.
Thinking on this, what is particularly confusing about it is that I am pretty much ambidextrous and almost anything else that I do I can do adequately with my off hand (I'm also left eye dominant so while nominally right handed a lot of two handed things, shooting, pool, etc. I do left handed). I can only presume that it's so hard coded in to muscle memory to get on from the left that when I try from the right it's like I'm inventing the option for the very first time.