I tell my wife that I'm trans because someone up there made a mistake, and to make up for it, they made her trans too (I'm older than she is), so we'd have something in common. It makes us work together well physically... and we understand each other incredibly well.
I haven't come up with much else but that someone messed up, at least on my end. Genetics, hormones in utero, etc etc can all play parts in it... but all I know is I am who I am... and I like me.
Dean