First born, only boy? Could be where it all began. Hard to say. I am an only child to my father. He made it real clear, he wanted a boy that he could fish with....only thing is, he didn't fish! As a girl, I never cut the mustard, although I never gave him one bit of a problem. Left home at 18 and never turned back.
My mother had another child, a boy, he is my mothers favorite, no doubt about if.
I feel that all parents have a favorite, most will deny it, but others see it clearly.
Have a good day!