Although NAFTA may seem to run contrary to the United States' short term interests, the global economic benefits will eventually cycle around benefit our economy. As Royobre pointed out, NAFTA is allowing America's wealth to flow into Central and South American nations, which is in turn stimulating local economies in those nations. Accordingly, they are developing far more rapidly than they would without NAFTA. Eventually, they will become trading partners with the United States, lifting our own economy.
More importantly, NAFTA is paving the way for economic globalization, and is a key step in a paradigm shift that will allow the entire world to rise to American economic standards, and will also result in something of an enlightenment in 2nd and 3rd world countries, eventually reducing violence and terrorism.
That said, the American government does have a duty to protect American jobs, and NAFTA currently does not uphold that standard.
Perhaps we could find a balance.