Yup yup, it realeases chemicals in the brain. People can harp all they want about it being about love, but love is a chemical too- technically, we as humans are meant to remate every four years to procreate. Ever wonder why you have to "refall" in love? The seven year itch? etc. There are lots and lots of websites on this and information.

That said- if the boyfriend is forcing it, pushing, guilt tripping her, then it needs to stop.