Demand for gas at the pump is notoriously fickle -- it is very rigid in the short term, but very flexible in the long term. Once you've decided to make a trip, it's hard to change your mind. I think that's why big fluctuations in gas prices are really annoying.
(I think another part of the reason is that they are prominently displayed all around town -- the price of a dozen eggs or a gallon of milk varies similarly, but the prices aren't posted on billboards.)
See, I really wish prices would go up to about $4.00/gal or more -- that's about where people really start to change their behavior -- but stable, so that people could plan better. I'd be all for government regulating prices to be high (keeping the difference through taxes) and stable, so that these swings wouldn't cause so much economic uncertainty.