"And Yitzchak came from the vicinity of the well of Lachai-ro'i..."(24;62)
Rashi comments: "Since he went to bring Hagar to Avraham his father, so he could marry her." Indeed, that is the place where Hagar was miraculously saved (16;14). And afterwards, when Avraham marries his second wife, Ketura (25;1), Rashi quotes the Midrash (Bereishit Rabba, 61;5) which identifies her as Hagar.
On the surface, a perfect family idyll. The father sends his slave to Aram-Naharayim, to find a wife for the poor orphan, and at the same time, the son looks for someone to sooth his aged parent's grief. Each caring for the other - surely the most beautiful example of true family life.
And so it is. But, without diminishing at all the said above, the asymmetry between the two is striking.
Avraham is old, and venerable; but God has blessed him in everything (24;1). Although he is a widower, after a long, full life, he needs nothing more for himself, save one thing - to see Yitzchak, his son and heir, settle down and build his own family. He probably yearns for grandchildren more than for new children of his own, and perhaps wishes to see the first stage of the promise "...it is through Yitzchak that offspring shall be continued for you" (21;13) fulfilled.
Avraham acts with the true love of a father, with energy, determination and generosity. But he is doing only the natural, obvious thing to do. On the other hand, Yitzchak's motive is not so clear. Why should his father need a new wife? The Torah itself testifies he needs nothing more! And even if he does - why Hagar, his deceased mother's bitter rival? Isn't this a slight to her blessed memory?
Perhaps, the need was not so much in Avraham's re-marriage, but in this reconciliation with Hagar. Avraham has led a long, full life, and has passed many trials; but only once did the Torah tell us that he was actually troubled - when banishing Hagar and Ishmael. God himself needs to sooth him: "...be not distressed over the boy or your maidservant..." (21;12).
And surely, there was a good reason to be troubled. At the very start of the conflict between Sarah and Hagar, the Ramban comments: "Our mother committed a grave sin in this, and Avraham as well in allowing her to do so" (16;7). Avraham passed through great trials - leaving his homeland and family, circumcising himself, offering his son on the altar - but this is the one scar left on his soul. Yitzchak is not so much giving his father a comfort in his old age, as he is relieving his conscience of this one burden.
By doing so, he is not slighting his mother, but to the contrary - he is removing the one stain from her memory. The only wrong she has ever done was basically for his sake - and he voluntarily relinquishes the advantages it gave him (for Hagar cannot be recalled without Ishmael being rehabilitated), thus atoning for it. Because true filial duty is not in covering up one's parents' faults - but in redeeming them, and setting them right. And the praiseworthy behavior of the son, is the true, ultimate triumph of the mother.