What makes a pasta shape pair with a sauce?
Fats adhere to broad or flat areas nicely (fettucini, linguine) and press the creamier sauces against more tongue surface to enhance/emphasize their smoothness
Pooling sauces needs nested, medium pasta (round or flat) that help to punctuate the sharper and more diverse flavors of a smooth, acidic by alternating between pasta and sauce
Angel hair and other fine pasta do well with a chunky sauce as it is effective with pick up but doesn't obstruct the sauce's texture once you get it in your mouth


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.


This book has a listing of specific pairings of sauces to pasta.
This book, upon which Geometry was based, also has a listing of
sauces with additional options.


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.