Its mostly economics...

When the market is under duress, OPEC lowers the price per barrel to try and sustain a certain level of consumption. When the market is healthy and vibrant, the price goes up because there's money flowing in the general market and people spend it.

Plus, gasoline/fuel prices are considered by economists to be "elastic." Meaning, prices can go way up or down before it effects consumption. People need gas in their cars to go to work, live their lives, etc. So while we bitch about, we still swipe our credit cards and fill her up.

An example of inelastic product would be food/dining out. If the economy's down, more people eat at cheaper places than more expensive ones OR, they eat out less.

There's tons of options with food/dining out. Not so many when it comes to purchasing fuel for our cars.