I'm going to go with the following:
Dogs are usually emotionally dependent on their owners. Some dogs are very intelligent, though that is not my point. Dogs thrive on companionship and usually accept their rightful position as subjects to their owner's discipline and command. I'm sure they understand when they're being respected and disrespected, but I think it's safe to say they assume a position of subordination with respect to their owners.
Cats on the other hand, are usually much more emotionally independent of their owners. They can also be very intelligent, though are not bound (do not depend) on their owners for companionship. As a result, they can be viewed as "less playful" and "boring." In actuality, they simply do not assume the same level of emotional subordination to their owners as dogs. Obviously it depends on the specific cat, but I have seen cats more playful with people than some dogs. Some are also not very interested in people and prefer other activities, such as catching birds and other animals in the backyard as it's just that much more exciting (think about it--what would you rather do if you were an animal?).
Whether a person likes dogs or cats is pretty obviously usually rooted in their childhood family experiences. Regardless of the cause, I think relationships between cat-owners and dog-owners and their respective pets are very different for the above reasons, and as a result, the 2 types of pet owners often do not understand each other. Whereas some owners prefer (or are used to) the subordinative emotional attachment of their pets, other prefer a companionship on a different level which may appear non-existent to some which haven't experienced it.
I have been a cat owner my entire childhood, but will most likely raise a dog as a pet sometime after I settle down in the next 2-3 years.