As has been stated already, basic economics goes a long way to explain the rise in gas prices. Other factors include:

No new refineries in 30 years.
Assinine government regulations that require dozens of different 'blends' to be refined (this is why gas prices go up in summer, they must refine multiple summer blends... that and demand goes up in summer).
Gas taxes (not that much, but it adds up)
New demand from india and china

Oooooooorr you could just blame Bush for it, if you don't feel like thinking.