I could tell you why, but Thomas Friedman can tell you better. This is no hippie cry to destroy Detroit and put everyone on bicycles; Friedman offers a hardnosed geopolitical argument that would make Kissinger and Brzezinski proud:
A gas tax reduces gasoline demand and keeps dollars in America, dries up funding for terrorists and reduces the clout of Iran and Russia at a time when Obama will be looking for greater leverage against petro-dictatorships. It reduces our current account deficit, which strengthens the dollar. It reduces U.S. carbon emissions driving climate change, which means more global respect for America. And it increases the incentives for U.S. innovation on clean cars and clean-tech [Thomas Friedman, "Win, Win, Win, Win, Win," New York Times, 2008.12.28].
Of course, if you think driving your Hummer with pride is more important than standing up to Iran and Russia... well, that's the sort of vanity that destroys empires.
Man up, America: gas tax and Smart cars are the path to a second American century.