Friend of The Food Geek, Craig, asked via Twitter:
@thefoodgeek Why does spaghetti taste better the next day? Does the refrigeration do anything to the sauce?
Refrigeration isn't the answer, per se, to this. It's rare for refrigeration to change flavors too often, as the refrigerator is more about slowing chemical reactions. Its job is primarily to preserve flavors. Though in the case of some tropical fruits, refrigeration can affect how they store and cause them to spoil inappropriately. Not the case with spaghetti, however.
As an American, I grew up making spaghetti this way: boil the spaghetti, make or heat the sauce, combine in a bowl, add some cheese. This is all well and good, but the Italians do it a slightly different way, and restaurants have some variations as well.
In the Italian method, you make your spaghetti sauce and have it ready in a pan. When you make your spaghetti, you pull it out of the water about a minute before it's at the consistency you want to eat it, then you add the spaghetti to the pan with the sauce. You don't use a lot of sauce, but more like if you were adding dressing to a tossed salad. Enough to coat and have a bit left over, not enough to be a major component of the dish.
In this scenario, the sauce finishes cooking the spaghetti, and the sauce actually flavors the spaghetti itself. Also important is that there's a lot of starch in the spaghetti itself and clinging to the surface of the spaghetti, and this additional cooking causes the starches to mix in with the spaghetti, changing the flavor a bit and the texture more. Starches don't have flavor in and of themselves, but they modify intense flavors by spreading them out, allowing you to experience more nuances of the flavors they modify. Reheating pasta essentially works this way, which is why you get better flavor from reheated spaghetti.
In restaurant cooking, a lot of pasta is made in the same water over the course of the day, and the water becomes infused with this sauce. If a sauce is too thick, the chefs will spoon some of this starchy water into the sauce, which does magical things to the flavor and texture. This is hard to do at home, though there is a way.
Harold McGee wrote an article about cooking pasta in a small amount of water, which is the opposite of conventional wisdom. The pasta cooked with a minimal amount water gives you water with a lot of starch, and that can be used to make your spaghetti taste even better before the reheating.