U could ask why some people have green eyes, why some are double-jointed, why some have red hair, why some don't like asparagus, why some like ice curling.
.. The world is filled w/ people of all shapes, colors & sizes, & they like (& dislike) all sorts of things. I'd be dubious of any scientist who said sexual orientation - gay, straight, bisexual
, etc. - is a result of any balance of chemicals.