Rav Reichman spoke about how Avraham's middah was that of chesed, but in order for Yitzchak to be born, Avraham had to temper his chesed with din, through the mitzvah of bris millah. Avraham had to learn that unrestrained kindness is not good either.
It is interesting that a person is born with a certain set of middos, and then has to work in their life to chanel these middos and to learn to use them in the right way. I have heard that when one is working on themselves they should start by trying to strenthen themselves in the areas that their middos are already strong, so that they will have one great middah through which to face all types of situations. Avraham may have been born with a predisposition towards chesed, but he must have also worked hard to maximize this middah. Yet, at the age of 100, a point in a person's life the they hardly feel like they can still change themselves, Avraham is asked to stop working on chesed and to now work on adding din to his repetoire. Avraham is told to do this directly by Hashem through the command of bris millah. I wonder how the rest of us can know at what point to stop trying to work on our one strongest middah and to temper it with the oposite middah. Of course, most people are not people of pure din or chesed, like the avot or imahot were, and we are constantly trying to ballance the two. Yet most people don't think to incorperate the middah which is their exact opposite. Should we be following this model of Avraham in our own tikun hamiddos? and if so, how?