Adam Clarke on Matthew 18:21

Till seven times? - Though seven was a number of perfection among the Hebrews, and often meant much more than the units in it imply, yet it is evident that Peter uses it here in its plain literal sense, as our Lord's words sufficiently testify. It was a maxim among the Jews never to forgive more than thrice: Peter enlarges this charity more than one half; and our Lord makes even his enlargement septuple, see Mat 18:22. Revenge is natural to man, i.e. man is naturally a vindictive being, and, in consequence, nothing is more difficult to him than forgiveness of injuries.