"Olam Chessed Yibaneh", the world is built up with kindness. Avraham Avinu was the paragon of Chessed, the fourth Holy Attribute with which Hashem governs the world. To accomplish his life's purpose, however, he needed to make Chessed manifest in this world through the Sephira of Malchus, for it is the nature of Chessed to eventually dissipate. The trouble was that there are five other Holy Attributes between Chessed and Malchus. How can Chessed exist without restraint, Gevurah? How can it express itself without assertiveness, Netzach? Indeed, Chessed can have no lasting presence unless it builds up a proper receptacle.
Finding a befitting receptacle, however is easier said than done. He tried to do it through his nephew, Lot, and through the many converts that escorted him. He gave and gave, but they could not contain such a degree of holiness. Like Hagar, they viewed the greatness they achieved as coming from themselves and thereby lost it. Only when he reached a level of giving that no existing receptacle could contain, was he commanded to remove the extraneous earthliness that had prevented Yitzchak from being born. Only then was the receptacle established and his Chessed found a resting place.
Adapted from Rabbi Vali's book on Bereshis. Questions and subscriptions can be mailed to: the Yeshiva