As the Jewish people headed into exile after the destruction of the Beis haMikdash, they passed the grave of our matriarch Rachel. The Midrash poignantly captures the image of Rachel's spirit weeping as she watches her children departing from Eretz Yisrael (incidentally, it is my daughter Rachel's Hebrew birthday). Why specifically is it Rachel who cries more than any of the other matriarchs? There is a small sefer "Reshimos" containing thoughts of R' Noson Wachtfogel and in the volume on tefilah he addresses this question. We know that Leah is described in the Torah as having "soft eyes" (einey Leah rakos), which Chazal attribute to her crying. Leah knew she was destined to marry Eisav, but through her tefillah and crying she changed that decree and merited to marry Ya'akov. We never find Rachel similarly described as crying. The crying of Rachel for her children is to make up for the crying she was spared in her lifetime.
R' Wachtfogel adds that we see another remarkable insight from this episode. Leah suffered perhaps a few years of crying in her lifetime, but Rachel has been crying for generations for the return of all her children from galus! We learn from here the tremendous power of tears and tefilah while one is alive and grappling with the challenges of this world.
During the three weeks there is a special emphasis on reciting tikun chatzos, esp. the section for tikun Rachel, as we hope for the geulah which will bring Rachel's crying to a close.