Musar על ברכות 9:14

The afflictions which are to be endured by the recipients of these three gifts of G–d are what are known as יסורים של אהבה, afflictions which demonstrate G–d's love for the person who undergoes them. Their purpose is to thoroughly refine such a person spiritually so that when the holy gift is received it will not reject the recipient [much as a transplanted physical organ may be rejected by the body that it has been implanted in. Ed.] This is why we are told in Shabbat 89 that in the future Israel will say concerning Isaac "he is our father!" This seems very strange at first glance, for Isaac personifies strictness, the מדת הדין. I have explained at length in my commentary on פרשת וירא that it was the very fact that Isaac could be judged by this standard that enabled G–d to apply the attribute of Mercy to people of lesser stature.