When people look at a description of someone or something, some people focus on the positive while others focus on the negative. The Torah states that Leah's eyes were "רכות" (בראשית כ"ט:י"ז). Rashi (שם בד"ה רכות), who bases his opinion on the Gemara in Bava Basra (דף קכ"ג.), says that this means that Leah's eyes were unattractive because of her weeping. She was weeping because since she was the older sister, she heard that she was intended as the wife for Eisav, and she didn't want to marry him. But is it clear that Leah's eyes were really unattractive? According to the Da'as Zekeinim MiBa'alei HaTosafos (בד"ה ועינישם ), Leah's eyes were actually beautiful; the word "רכות" means soft and because of those eyes, she looked young. Ibn Ezra (שם בד"ה רכות) quotes a view that the word "רכות" is missing an "א", and should read ארכות, long. Others say that it means simply that Leah's eyes were sensitive; this is why Rochel went out to the sheep and not Leah because her eyes were sensitive to the wind and this is the reason why Yaakov saw her first.
Why did Yaakov actually choose the younger sister Rochel, especially if it's true that Leah was indeed beautiful? Some suggest that he too heard the rumor that the older sister was for Eisav. If Yaakov would marry her, Eisav would say that Yaakov took not only his birthright, but he took his wife also. But Leah was determined to marry Yaakov. Chazal say in the Midrash that when Yaakov asked Lavan "Why have you cheated me?" Leah responded, "but you cheated your father when you identified yourself as Eisav, his firstborn" (שם כ"ז:כ"א). It is not that Leah was standing up for Eisav; she was actually standing up for herself and her desire to marry Yaakov. She thus said, "I was originally intended for the first born, but after you bought the birthright, you also acquired me through that action."
The Sefer Ma'aseih Hashem notes that when Lavan said that it was customary to marry off the older child before the younger one, first, he didn't use the words "גדולה", older, and קטנה"", younger. Instead, he specifically said "בכירה" and "צעירה," words which imply not exactly older and younger, but earlier and later born (שם כ"ט:כ"ו). Lavan was thus telling him that while maybe Yaakov's custom was that the title of "בכירה," implying the first-born status, can change hands, according to Lavan it can't. Yaakov traded in order to receive that status, but Lavan didn't agree that this was a status that could change hands. This may be what Lavan meant by saying that "it is not done this way in our place" (שם). In other words, the status of בכירה can't change hands according to him. According to Leah, however, it could and it did. And she was thus justified in marrying Yaakov, no matter what her eyes looked like.