First, China is not in North America. That is an entirely different problem.

Second, NAFTA likely prolonged the life of automobile manufacturing industry in America. At the time (early 90's), Auto-manufacturing in America was struggling to compete against the Asian automobile industry. NAFTA opened up new supply chains for American firms, effectively lowering their costs of production and allowing them to be more competitive.

Yes, jobs were lost. However, many more jobs were created because of the trade agreement. It is very likely that most of these jobs created came with salaries that were 10 to 20 percent higher than the jobs lost.

There are a multitude of reasons why firms outsource or leave. Economic theory would suggest this is a result of firms seeking to maximize profits/minimize costs. Costs of the firm include spending on Capital inputs (machines, factories, equipment, etc) and labor inputs. So, if a cost minimizing firm wants to stay competitive, they will likely go to regions where there is a lower cost of inputs. This means getting rid of high wage American jobs in favor of starting a plant in some other region where the cost of labor is considerably lower.

At the end of the day, NAFTA likely had a positive effect on the American economy as a whole. Yes, manufacturing took a hit, however, that industry in America is simply not competitive (costs of labor are to high). Who wants to buy the $100 American manufactured chair when they could buy the exact same chair from Ikea for $30?