I had cats for as long as I lived, I had at least one so from experience cats don't quite get very affectionate...at least not for long periods of time. Cats have the tendency to attach to the house and perceive the masters as people they own :-). When they rub their heads to you, they actually leave their scent so you are marked as hers/his :-). BTW, you know they purr if they are in pain as well as when they are happy?
Regarding the question, food is a way to make it more affectionate, also for some of my cats brushing and playing had the same effect. You could try doing a little massage on her/his back while your petting her.
Hope this helps