Every White House and congressional commission looking at these threats has called for more human intelligence. Its importance remains undiminished. Research shows that the motives are mostly personal, tawdry and banal and mainly involve money. What is interesting is how cheaply spies sell themselves. If true, he bargained better than most.
Revenge comes in second as a motive.
Illicit affairs are sometimes a factor, but blackmail doesn't promote long-term productivity. For some, the sheer thrill of living a secret life, on the edge, is a narcotic. With risk comes intensity. Some want to prove that they are smarter than everyone else. It makes for mistakes.