Changes in gasoline and diesel prices mirror changes in crude oil prices. Those changes are determined in the global crude oil market by the worldwide demand for and supply of crude oil. Weak economic conditions in the U.S. and around the world in 2008 and into 2009 led to less demand which helped push prices down.
With the worldwide economic recovery underway, demand is on the rise again but unrest in the Middle East and North Africa has put supplies at risk. This combination of rising demand and reduced supply helped to push prices higher over the last few years. However, the recent downturn in prices was the result of the growth in oil supplies, largely from the U.S., outpacing the growth in global demand.
What's Up with Gasoline Prices? charts are available in high resolution versions on the Gas Prices Explained website.