It's been said that currency markets are affected by the price of crude oil. Currencies such as the U.S. Dollar typically react to the rise and fail of the price of this commodity. Why are they linked so closely?
Because we need energy in the form of oil. If more of our money is spent on oil, there is less money to spend on other items especially luxuries like dining out and new cars (ironically)
Since there is less money available, the price of other things shift with it and the whole economy moves. Since less money is available, the value of a single dollar goes up.
Basically, it is because we as a species (let alone nations) are unbelievably dependent on having oil at this point in our existence.