A dog is called Man's best friend because they are much more faithful and understanding then cats. To a dog you mean far more to them then you would to a cat.
Cats don't care about you, cats are basically selfish. Of course there are exceptions. This is not to say that cats or other pets aren't faithful but just not as much as a dog.
"A dog's social structure is very similar to base human tribal social structure and the affection they display (designed to strengthen the pack) is compatible with the human notion of friendship (which is one of the ways human social groups are formed)."
An experiment was done to see whether or not cats have an emotional commitment with its owner.
This experiment is definitely faulty because it only tested one cat, but this shows how cats will likely react because they are used to being alone/independent and don't need that commitment with a person. They only seem as if they do for their own benefit of food and such. However, dogs thrive on that commitment with that special someone.
(Hachi: A Dog's Tale is a beautiful movie which showcases the relationship between a man and his pet dog.)
In conclusion, "man's best friend" basically means someone who is your (faithful) friend. The majority of dogs show exactly that. But agian some cats could also have that real, special connection.
And besides how can you say no to a face like this....
....even though they just might be using you and don't really love you :p teehee