Dogs are pack animals. The prefer living in groups. In each pack there is an alpha male and the rest are subordinates and generally do not challenge authority. Humans are the alpha's or pack leaders, and the dogs take a subordinate role and do as they are told and they love company. They protect their owners too. (Obviously not always otherwise Cesar Milan would be out of a job!)
Dogs are loyal, obedient, can be easily trained, and will, in most cases do as they are told. This makes them man's best friend.
Cats on the other hand do not live in groups. The only "packs" of cats are lions, all other cats are solitary animals, coming together only to mate. You cant really train a cat, it does whatever it wants. You never really "own" a cat, it tolerates you.
Cats are not man's best friend. Man is a cats best friend. When it suits them that is.