 Why does Paul think aliens are watching Earth?
As a result of them the permanent population includes smugglers and black-marketeers, fugitives from justice and international con men, espionage and counter-espionage agents, homosexuals, nymphomaniacs, alcoholics, drug addicts, displaced persons, ex-royalty, and subversives of every flavor. Local law limits the activities of few of these. Paul said, "What ever happened to those poxy flying saucers?" And when he looked blank, I added, "The Flying Saucers." "Well, hell, suppose for instance there's this big Galactic League of civilized planets. You're not eligible for membership until you, well, say until you've developed space flight. Meanwhile, they send secret missions down from time to time to keep an eye on your progress."
 The aliens are watching Earth's civilization go through wars and struggles as a form of amusement.
 The aliens are watching Earth's civilization go through wars and struggles as a form of amusement.  They want to invite Earth to join the Galactic League of civilized planets.  Man has invented the H-Bomb. The aliens are scared.  The aliens are preparing to harvest humans as a food source.