Short version: commodity prices are determined by supply and demand, both physical demand (from people who produce the oil or people who use the cotton) and financial demand (from traders and investors who have no particular use for soybeans or alumina, but see opportunity to make money by buying, selling, shorting, writing derivatives on, the commodity). For oil, there is debate over how much the financial markets affect oil prices. Gasoline price movements primarily reflect oil prices, but also refinery capacity and other local conditions. Local taxes will affect local retail gasoline prices, but since taxes don't change every day, they aren't causing the changes in gasoline prices. Short of starting a war in the Mideast, the US govt doesn't have much ability to manipulate oil or gasoline prices. The President sometimes orders a strategic reserve release, when oil prices are very high, it can bloody some financial speculators but seldom has any large or lasting effect on oil prices. All this results in rapid and sometimes puzzling volatility in oil and gasoline prices. Just like stock prices are constantly changing, and sometimes you don't know why.
See this for more information.
http://www.eia.gov/pressroom/presentations/newell_05052011.pdf