Why does the Torah insert the word "Gam" (the second time)?
Rashbam: By inverting the words "Vaye'ehav" and "Gam", in which case the Torah is telling us that not only was Yakov intimate with Rachel too, but that he also loved her more than Le'ah. 3
Tosfos ha'Shalem (1, citing R. Efrayim): A double inclusion excludes. He did not love Leah at all; she was Senu'ah.
Tosfos ha'Shalem (4): It teaches that he loved also Rachel's Shifchah [after he married her] more than Leah. 4
See Sanhedrin, Daf 22b.
Because of her good deeds (Seforno). The Seforno does not however, learn this from the word "Gam", but from the fact that the Torah sees fit to mention that Yakov loved Rachel more than Le'ah.
See also Or ha'Chayim. Oznayim la'Torah: Others, playing on the word "mi'Le'ah", explain that he loved Rachel because she handed over the Simanim that he had given her to Le'ah to save her from shame and embarrassment (an incedible display of selflessness).
Why does the Torah insert the words "Od Sheva Shanim Acheros"?
Rashi: To teach us that, in spite of what Lavan had done to him, he worked with the same integrity throughout the second set of seven years as he did throughout the first. 1
Tosfos ha'Shalem (10, citing R. Efrayim): They were unlike the first seven years, which were like a few days. These seven years, each day he longed for them to end.
Presumably, this is precisely what Lavan had in mind when he used exactly the same words in Pasuk 27. (EC)
According to those who explain that Yakov truly hated Leah, it was forbidden to have Bi'ah with her (this is one of the nine Midos of despised relations), and Bnei Leah would be spiritually blemished!
Ran (Nedarim 20b DH Bnei Senu'ah): The Isur is only if he thinks of another woman at the time of Bi'ah.
Yakov did not hide his hatred. (Re'em, in Shitah Mekubetzes Nedarim 20b DH Acharei, says that the Isur is due to "Lo Sisna... bi'Lvavecha."
Ra'avad (Ba'alei ha'Nefesh Sha'ar ha'Kedushas, 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.