ויביאה יצחק האהלה שרה אמו ויקח את רבקה ותהי לו לאשה ויאהבה וינחם יצחק אחרי אמו
And Yitzchok brought her into the tent of Sarah his mother; he married Rivkah, she became his wife, and he loved her, and thus was Yitzchok consoled after his mother (Breishis 24:67).
Yitzchok Avinu remained disconsolate for years after Sarah Imeinu's death. He only emerged from his grief and found comfort upon marrying Rivkah Imeinu. At first glance Yitzchok's behavior is puzzling. While the Halachah mandates appropriate mourning, it frowns upon excessive mourning.
אל יתקשה אדם על מתו יותר מדאי וכו' שזה הוא מנהגו של עולם. והמצער עצמו על מנהגו של עולם הרי זה טפש
A person must not grieve over his dead excessively because this is the way of the world. And one who aggrieves himself about the way of the world is a fool. (Rambam Hilchos Avel 13:11)
The Torah has anticipated our question. The passuk recounts that Yitzchok brought Rivkah into the tent of his mother, Sarah. That is, in seeking a wife Yitzchok had not simply been searching for his personal soulmate. He sought a woman of valor who would continue and transmit the matriarchal masorah founded by Sarah Imeinu[1]. Yitzchok was rejuvenated from his personal grief over the loss of his mother. He had, however, been inconsolable over the suspension and potential loss of Sarah Imeinu's matriarchal masorah[2]. This grief was finally dispelled when Rivkah entered Sarah's tent, and proved a worthy successor.
[1] Regarding the matriarchal masorah, see Rav Soloveitchik's Tribute to the Rebbetzin of Talne in Tradition.
[2] For a similar distinction between grief for loss of Torah as opposed to grief for an individual see Tosafos Sanhedrin 68a, s.v. ve-hayah