Showing grace is incumbent on Allah, He had created people and furnished them with passion. If He does not show grace to them, that would mean alluring them, which is bad. In this case grace signifies furnishing arguments, developing intellect, sending Prophets during the period fixed for their sending and after their termination continuation of the Imams so that the chain of obtaining the objective may not be broken.
source : http://www.islamicecenter.com/e-library/logic_islamic_rules/logic_islamic_rules_makarem_subhani_03.html
