How did United States and Mexico get so far from one another?

I continue to be baffled by the break in harmony between United States and Mexico.  We appeared so close to each other in 2001 that TIME magazine called the border zone, Amexica. "A Whole New World," the article exclaimed.  

President Bush even went to Mexico, instead of Canada, for his first international trip.  The entire Mexican cabinet met with the entire U.S. cabinet days before September 11, 2001.  American and Mexican relations were closer than ever. 

After Sept. 11, Americans traveled in large numbers to Mexican resorts versus going to oversea destinations.  Mexico's travel campaign tag line was indeed correct:  Mexico: closer than ever. 

What happened?

How did we go from A Whole New World (TIME June 11, 2001)  to The Great Wall of America (TIME June 30, 2008)?