Petrov decided that, all else being equal, he would prefer not to destroy the world. He sent messages declaring the launch detection a false alarm, based solely on his personal belief that the US did not seem likely to start an attack using only five missiles.
Wikipedia disagrees that Petrov’s decision was solely based on whether the US would start a war with only 5 missiles, citing in addition the newness of the launch detection system, lack of corroboration from ground radar, and the message passing through verification too quickly:
Petrov later indicated that the influences on his decision included that he had been told a US strike would be all-out, so five missiles seemed an illogical start;[2] that the launch detection system was new and, in his view, not yet wholly trustworthy; that the message passed through 30 layers of verification too quickly;[15] and that ground radar failed to pick up corroborating evidence, even after minutes of delay.[16]
Wikipedia disagrees that Petrov’s decision was solely based on whether the US would start a war with only 5 missiles, citing in addition the newness of the launch detection system, lack of corroboration from ground radar, and the message passing through verification too quickly: