Why was Leah Senu'ah (hated)?
Targum Yonasan, Bechor Shor and Ramban (citing the Radak): Yaakov did not really hate her. 1 The Pasuk means that he did not love her as much as Rachel.
Seforno: Because Yaakov subsequently saw signs that she was barren, and thought that that is why she agreed to trick him. 2
Ramban: Because she willingly set out to trick him. Even if one were to claim that she was simply fulfilling her father's orders, Yaakov considered her guilty of not subsequently informing him, or at least of dropping a hint, that she was Leah. 3
Tosfos ha'Shalem (to 29:30 (8)): Yaakov thought that if he shows love for her, Esav will hate him doubly. 4
Bava Basra 123a: Esav's deeds were hateful to her (people were saying that she will marry Esav).
Ohr ha'Chayim (based on Bereishis Rabah 71:2): People were saying she will marry the Sonei, i.e. Esav. She was called on his name.
This is evident from the previous Pasuk, 29:30 (see Keli Yakar there).
Even if she became pregnant right away, it is not recognized until about three months (PF). Ha'aros on Birkei Yosef (OC 240:2) - Hashem knew that her motivation was because she hated Esav's deeds. After she gave birth, Yaakov realized this and ceased to hate her.
Bereishis Rabah (71:2): When Yaakov saw how Leah had tricked him, he wanted to divorce her. What did Hashem do? He opened her womb and allowed her to have children, prompting Yaakov to say 'How can I divorce the mother of these children?'
Surely this is not a Heter to hate her! Perhaps he loved her, just he was careful not to show this openly. (PF)
Is being hated a reason to open Leah's womb?
Ramban: Indeed it was, since only Hashem knew that she acted the way she did because she genuinely wanted to marry the Tzadik Yaakov, so He took pity on her. (Refer to 29:31:1:3*). Hence the Pasuk writes, "And G-d saw" - He took pity on her so that Yaakov should not desert her. 3
Abarvenel (to Devarim 21:16): Yes - Hashem always favors the hated wife, and the Bechor is from her.
Ohr ha'Chayim: If she would be barren this would support that she is really Esav's Zivug. Therefore Hashem opened her womb, and made Rachel barren -perhaps this would remove the hatred!
Malbim: All the Imahos were barren, so they [and the Avos] would pray, and the birth would be miraculous, through Hashgachah. Hashem saw that she is hated, and Yaakov will pray only for Rachel, therefore he opened her womb, and Rachel, Ikarah (the primary wife) Shel Bayis, remained Akarah (barren).
Oznayim la'Torah: If her father was a trickster, Yaakov held it against Leah for following in his footsteps - so he hated her. But Hashem knew that Leah did what she did solely to escape falling into the clutches of Esav ha'Rasha - so he opened her womb and closed that of Rachel, to demonstrate to Yaakov, who knew that he was destined to father twelve tribes, that Leah was no less his Bashert than Rachel, and that the incident was His doing, not Leah's.
Like we find by a Sotah. If she is innocent, she gives birth to compensate for her disgrace, including suspicion of her. Tosfos ha'Shalem (8) - Yaakov refrained from her, amidst concern lest she was intimate with other men, just like she tricked him.
See also Seforno to 29:33.
Why were Avraham and Yitzchak initially barren, but not Yaakov?
According to those who explain that Yaakov truly hated Leah, it was forbidden to have Bi'ah with her (this is one of the nine Midos of despised relations - Nedarim 20b), and Bnei Leah would be spiritually blemished!
Ran (to Nedarim 20b DH Bnei Senu'ah): The Isur is only if he thinks of another woman at the time of Bi'ah. 1
Yaakov did not hide his hatred. (Re'em, in Shitah Mekubetzes to Nedarim 20b DH Acharei, says that the Isur is due to "Lo Sisna... bi'Levavecha" (Vayikra 19:17).)
Ra'avad (Sha'ar ha'Kedushah, cited in Birkei Yosef OC 240:2): At a time of Bi'ah she was pleasing to him (then it is permitted).
Refer to 29:25:154:3.
Oznayim la'Torah: In any event, Leah became pregnant after Yaakov had been intimate with Rachel - as is evident from the juxtaposition of this Pasuk to the previous one. Consequently, Reuven was not a 'ben Temurah' (when a man is intimate with his wife and thinks of another woman - Nedarim ibid). Compare to 29:25:154:1, and the note there.