The Volokh Conspiracy's David Bernstein invokes a standard argument for America's superiority over her Canadian and European allies: the direction of emigration is primarily towards America from these other countries, rather than vice versa. For all I know, the numbers may back him up, but his reasoning is faulty.
Consider, for example, cemeteries. By Bernstein's reasoning, they are truly magnificent places--after all, people move into them at a prodigious rate, but the number exiting is negligible. Obviously then, the people therein must be content, while those elsewhere pine for its comforts.
A similar bias infects Bernstein's argument about countries. America is famous as a "land of opportunity"--a place where the ambitious can go and improve their lot through hard work. Other countries promise instead a safe, stable, comfortable life, with fewer demands and smaller maximal rewards. The result? The ambitious from all over the world--restless by nature--flock to America in search of opportunity, while the more easily contented, by definition, stay where they are--whether they are in America or elsewhere (and whether they would be happier elsewhere or not). The result: a net influx into the US, which Bernstein mistakenly takes for a proof of America's preferability.