Yitzchak prayed to Hashem opposite his wife, for she was barren, and Hashem turned to him, and Rivka his wife conceived.
Yitzchak and Rivka were married for twenty years when he finally beseeched God with all of his heart for children. For the first ten years, Rivka was not yet old enough to bear children, so children were not something that they even considered. For the next ten years, they tried to have children, but were not blessed with them. It was only at the end of those ten years that Yitzchak finally reached his limit, and began to implore Hashem, begging for children. And he was answered. But why was it that he waited exactly ten years before requesting children?
Rashi explains that just as Avraham only married Hagar after attempting for ten years to conceive with Sarah, so did Yitzchak follow suit. The reason that Avraham waited ten years until he married Hagar, was because, as the Talmud teaches, “One who is married for ten years and has not had children must marry another wife or divorce his first wife,” so that he will be able to have children.
- David Kimchi, the Radak explains that the idea of taking a second wife is considered by the Torah to be a hurtful act from husband to wife. He explains that Avraham and Yaakov never considered doing such a thing, and were only willing to do so when it was brought up and encouraged by their wives. Yitzchak, he explains, did not do so despite his wife’s apparent barren state since he did not want to hurt his wife at all. Thus, it would seem that until then, Yitzchak was content to wait for Hashem to send him children at the right time. But after ten years, he grew desperate, for he did not want to divorce Rivka, nor did he want to hurt her by taking a second wife. It was then that his prayers took on a new dimension – he was asking Hashem to allow him to stay with his wife, Rivka, and to help him avoid causing her pain. And he was answered.
The Midrash tells a remarkable story that occurred in the city of Tzidon. A couple was married for ten years, and they were still not blessed with children. The husband wondered if perhaps the time had come for them to split up and seek new spouses, since they both greatly desired to have children. The heartbroken couple came before R. Shimon bar Yochai, requesting his advice. R. Shimon bar Yochai agreed with them that they ought to split up, but he said, “Before you do, make a party. Just as when you married, you did so with food and drink at a party, so, too, should you split up in a similar way.”
Before this party, where they were to say goodbye, the husband told his wife, “Before you leave to your father’s house, you may take anything at all that you wish from my home.”
At the party, the wife got her husband drunk to the point where he collapsed. Then she told her servants to pick him up and take him home with her. The man woke up in the middle of the night, shocked.
“Where am I?” he demanded to know.
“You are in my father’s house with me,” said the woman. “Don’t you remember telling me that I could take anything that I wanted home with me? The only thing in the world that I want is you.” The couple went back to R. Shimon bar Yochai, and when he heard this story, he blessed them with a child, and his blessing came true.
Why did the Rabbi not bless them with a child right away? Why did he need to wait for this story to play out? The answer is that the birth of a child is something that is to come from deep love between spouses. Only where this is that deep love should a child really come to this world. The Talmud teaches of the terrible effects on a child’s soul that are brought about by parents who are not dedicated to one another during the conception of that child. When these spouses demonstrated the incredible love that bound them, R. Shimon bar Yochai then saw that, indeed, this is precisely where children are to come from. The very party that the Rabbi suggested to mark the end of their marriage is what allowed them the eye-opening experience that they had, and the opportunity to express their love. This brought them a child.
The birth of a child is an event that brings incredible unity to a couple. “Therefore, a man should leave his father and mother and cleave to his wife and they will be one flesh.” Rashi explains that the couple becoming one flesh is accomplished when they have a child, who is one body that was created by both parents together. Only in a child do parents create a tangible being out of their unity.
Only when Yitzchak felt such love for his wife, and wanted so deeply to remain partners with her in life, did he truly express what was needed for Hashem to give him those children. And thus, though he certainly mentioned his desire for children in his prayers in years past, it was when he begged Hashem that he not be forced to hurt his wife, that he expressed the requisite love and devotion needed to turn him and Rivka into “one flesh,” in the children that would be born from them, that his prayers merited to be answered, at last.
 Rashi to Bereishis 25:26 s.v. ben
 Bereishis 16:3
 Yevamos 64a
 Commentary to Malachi 2:10
 Yalkut Shimoni Bereishis 16
 See Zayis Raanan (of the Magen Avraham) to the Yalkut Shimoni, ad loc.
 See Nedarim 20b, and Shulchan Aruch Orach Chaim 240
 Bereishis 2:24
 Ad loc., based upon Sanhedrin 58a. See also Tiferes Yehonason (of R. Y. Eyebshutz) to Parshas Emor 21:13, regarding why a Kohen Gadol should not marry a widow or divorcee, where he speaks about this verse and also connects the coming together of parents as one flesh in a fascinating way with the nature of their children.