Pumping air into wine (through an aerator) oxidizes it, which in turn creates new flavor compounds. In addition, as the wine is exposed to oxygen for a longer period of time, less sorbitol is formed; this higher oxidation ratio helps prevent the formation of sulfites that can cause astringency and harsh flavors. Finally then, because there’s more space between all the molecules through oxidation and they are not bound together so tightly anymore (i.e., due to lower pH), wines aerated at home will usually taste smoother and more mellow than wines that are not!