The hormone oxytocin is mainly responsible for motherly love. This hormone engenders the love within mothers for their children. I have recently read an article on how this hormone is linked to love for pets. But, I am uncertain about the boyfriend part of your question.
It pretty much comes down to a motherly thing, but maybe not so much in the boyfriend category. That's more of the fetish stuff. I have a boyfriend and I treat him like more of a friend that I fuck with, honestly XD
