Probably the best single predictor of whether two people will be friends is how far apart they live.
If one person lives in New York, and another in London, it is unlikely that they will be friends.
Knowing that as a god, Hercules is immortal and invulnerable, Hades sends his two lackeys, Pain and Panic, to kidnap Hercules and turn him mortal by means of a magic potion.
However, the arrival of two mortals, Amphitryon and Alcmene, causes Hercules to miss the final drop of the potion, causing him to retain his godly strength.
Perhaps the most general answer is that we like people who reward us and who help to satisfy our needs.