Spaghetti goes well with tomato sauce with ground meat. Fettucini goes well with a creamy sauce. Why? What general principles about the characteristics of the pasta and the sauce make them work well together? When is angel hair superior to spaghetti, and why does macaroni work well with cheese?
My basis for this is (1) ease and volume of transport, plate to mouth of sauce; as well as (2) how the pasta interacts with the sauce to create mouth feel and churn of sauce against tongue. As for gnocchi and other variously shaped pasta; if it has a cavity, it should have fats, if it has a textured surface acidic sauces will cling nicely.
Obviously, I would shy away from pretending any of these were definitive answers, they are reasoned and seasoned, based on sauce/pasta dynamics and experience preparing them.
I don't seem to be able to find any good, objective maxims for when to use what; but, considering the variety of pastas and the prolific diversity of sauces (i.e. a different ragu for every house), it seems only appropriate that every person should have an opinion on why a particular sauce works with a particular pasta.