I think people are just born genetically with one side of the brain more dominant than the other depending on whether your parents are left or right handed.
I remember when I used to play drums with my brother. He would always start rolls with his right hand. My natural tendency was to start with my left.
So when we played drums together it was like looking in a mirror.
Even now I have to set up a drum set backwards in order to play it properly.

Only way I can figure it is that my father is left handed and my mother is right handed and we must have inherited a little from both, except it got kinda twisted.

Be happy that they make left handed guitars anyways
I've been looking for a left handed piano for years and they just don't make em.