What was the source of Rachel's jealousy of her sister Leah?
Rashi: She was jealous of her good deeds. Had her sister not been more righteous than her, she would not have merited children.
Tosfos ha'Shalem (1) #1: The words of her sister "my husband will accompany me" aroused her jealousy.
Tosfos ha'Shalem (1) #2, from Bereishis Rabah 71:6: she thought, because Leah married Yakov first, she merited children - and this was because I gave to her the Simanim!
Malbim: She understood that Hashgachah causes the hated wife to bear children. It is better that I be hated, and bear children!
Did Rachel 'blame' Yakov for her inability to have children?
What do we learn from Rachel's statement "v'Im Ayin Mesah Anochi"?
Ramban: What she meant was that, if Hashem does not give her children, she will die out of distress (in the way that women tend to exaggerate their pain).
Tosfos ha'Shalem (1) #1: For whom will I long and get delight from?!
Tosfos ha'Shalem (1) #2: Who will mention me after my death?
Tosfos ha'Shalem (#2): Due to these words, she died when giving birth to Binyamin. 3
Ha'Emek Davar: Sometimes a woman wants children for pleasure
As implied in Yeshayah, 56:3:
Tosfos ha'Shalem (6) supports this from Chanah's words "Hashem Memis u'Mechayeh" - He gave to me children, and revived me.
The curse of a Chacham is fulfilled, even if it was conditional and the Tenai was not fulfilled (Makos 11a). Perhaps this is why she died at the first moment that she was not destined to bear more children (PF).
Ha'Emek Davar: Chanah vowed that if Hashem gives to her a son, she will give him to the Mishkan, to show that she did not intend for pleasure.
Why does it say that she did not bear children "l'Yakov"?
Ohr ha'Chayim: Yakov already fathered children, so he is not sterile. She must be sterile!
Ohr ha'Chayim #2: He is a great Tzadik, so her lack of children is due to him.
Malbim: She attributed her lack of children to him. Scientists say that excessive love sometimes prevents bearing children. Or, Hashgachah causes the hated wife to bear children.
Ha'Emek Davar: She thought that he did not merit children from her. Even though he had children from Leah, Rachel was his primary wife. If so, he did not merit, and he should be Moser Nefesh to do something to merit [children from her].
Why does it say that she was jealous "b'Achosah"?
Ohr ha'Chayim: Sisters normally have similar natures. Had a stranger given birth, she could attribute this to her nature.
Malbim: Yakov already fathered children, so he is not sterile. She must be sterile!