Avrohom Avinu was the greatest Ba'al Chesed to ever live. His life was a continuous song of Chesed to others. Yet, in the Torah we find one event singled out as the epitome of his Chesed: that of serving the three guests on the third day after his Mila. The Baalei Mussar all try to find the single factor in this event that made it so monumental that it was recorded so famously above all the others.
Rav Gedalia Eisman (see U'Lisitcha Elyon) the Mashgiach of Kol Torah says that after the old and weak Avrohom came to greet the "guests" and made his generous offer, they brazenly answered (Vayeira 18:5) "Kein Taaseh Ka'asher Dibarta." They didn't graciously accept the offer, but, rather, commanded Avrohom to do exactly as he promised.
Everyone gets a geshmack out of helping others from the goodness of his own heart. But if instead of appreciation and recognition for his good deed, the recipient turns around and demands to be served, the bubble is burst and all excitement over performing the chesed dissipates and even turns sour. Pangs of anger well up inside of us as a natural response to the insolence of the person we wanted to help.
Not so Avrohom Avinu whose response to this arrogant answer was, "Vayimaher Avrohom HaOhela El Sarah." Avrohom did not care about the atittude of the recipient, only about helping him with his needs. It made no difference if the recipient was gracious or obnoxious. On the contrary, if the recipient demanded of Avrohom, that was a clear sign to him that the recipient was in dire need and should be helped faster and harder.
Avrohom, the symbol of Chesed's response to the Malachim's demand, was to run to fulfill this demand. That is the path Avrohom set forth for us, and that is the ideal we should each strive for when it comes to Chesed.