Why do Chazal offer a derush to explain the name Reuvain when the pasuk (29:32) tells us very clearly why Leah gave him that name – Leah was saying, “Ra’ah Hashem b’onyi,” Hashem saw her pain and gave her a child?
If you look at the way the pesukim describe the names of Leah’s other children, you will see something interesting. I’ll work backwards:
“Va’tomer ha’pa’am odesh es Hashem, al kein kar’ah shemo Yehudah.” (29:35)
“Va’tomer ha’pa’am yilaveh ishi eilai… al kein kara’ah shemo Levi.” (29:34)
“Va’tomer ki shama Hashem ki senu’ah anochi… vatikra es shemo Shimon.” (29:33)
“Va’tikra es shemo Reuvain ki amrah ki ra’ah Hashem b’onyi…” (29:32)
By Yehudah, Levi, and Shimon, first the Torah first tells us the reason behind the name and then the name. When it comes to Reuvain, first we are given the name, and only then the reason. The Torah's placement of the justification for the name only after the fact hints that there was another hidden reason that had already led Leah to already choose the name Reuvain. That reason is revealed by Chazal.
We find inother cases as well that there is an overt reason given for the name chosen, but behind the scenes there is a subtext that reveals something different. When Rachel gives birth to a child she exclaims, “Asaf Elokim es cherpasi.” (30:23) The Torah then tells us that she named the child Yosef, saying, “Yosef Hashem li ben achier.” (30:24) Don’t we already have a reason for the name Rachel chose, i.e. “asaf Elokim es cherpasi?” And given that reason, wouldn’t it be more fitting for her child to be called Assaf rather than Yosef? Ksav Sofer explains that in her heart Rachel felt “asaf Elokim es cherpasi,” but she did not want to saddle her child with a name that would be a constant reminder of the embarrassment she suffered. Therefore, she used the name Yosef, “leimor…,” so that people should say, “Yosef Hashem li ben achier.”
Another example: the name Yis(as)char. Leah says she chose the name because “nasan Elokim sechari,” (30:18) yet one cannot help but hear overtones of “sachor sicharticha,” (30:17) the fact that she “rented” Ya’akov. Chasam Sofer writes that the reason we usually pronounce the name Yisachar and not Yisaschar is because “sachor sicharticha” is something to be kept private; it’s the hidden subtext. Only in this parsha, in the context of the name being given and sachor sichaticha being overtly mentioned, it should be pronounced Yisaschar.
Getting back to our starting point, why did Leah feel such a strong need to contrast the behavior of Reuvain with that of Eisav? Remember that Leah was actually destined to be the wife of Eisav, which is why (as Rashi explains) her eyes were sore from crying. Eisav should have been a helper to Ya’akov, supporting his study of Torah, enabling him to achieve spiritual success. Eisav, however, rejected that role completely. When Ya’akov dressed up as Eisav and took the brachos, Ya’akov in effect took on the role of Eisav in addition to his own. With that new identity as Eisav, explains the Sefas Emes (5647), came the relationship with Leah. The deception by Lavan in Leah’s marriage parallel’s Ya’akov’s “deception” to take on the role of Eisav. The relationship between Reuvain and Yosef, Leah’s children, parallels the relationship that should have existed between Eisav and Ya’akov. While Eisav rejected his role and failed in his mission, Leah’s children achieved success in theirs, validating their mother’s tikun of that role.