There has been a study conducted that measures the levels of Oxytocin in cats and dogs when they are exposed to their owner after a short period of seperation. Oxytocin is the hormone that is most closely associated with love and affection.
This study proved that Dogs are the more loving pet. They saw on average an increase of over 50% in Oxytocin levels after being united with their owner. Cats on the other hand only saw an average increase of 9% in Oxytocin levels. The researchers actually stated that most of the cats did not show any increase at all, while a few exhibited a 30-40% increase. I suppose it just depends on the cat.