"And Reuven went and he laid (with) Bilhah, the concubine of his father." (Bereishit 35:22)
The Gemara in Shabbat (55b) explains that Reuven did not sleep with Bilhah, he only switched around the beds. The Gemara explains that it was because of this action that the tribe of Reuven later stood on Har Eval (the mountain for curse, see Devarim 27:13). The Gemara brings proof of this from the Pasuk "Cursed is he who sleeps with his father's wife" (Devarim 27:20) which is in close proximity to the mention of Reuven standing on Har Eval. The Gemara goes explains further that the reason that Reuven moved the beds was to protect his mother's honor. Let us attempt to understand who Revuen was, what led him to move the beds, and how he changed afterwards.
We are introduced to Reuven when we are told the occasion of his birth: "Because God saw in my [Leah's] suffering, that now my husband shall be caused to love me" (Bereishit 29:32). The purpose of Reuven's birth is indicated in two ways. The instinctual purpose is as a reminder of suffering, while the effectual outcome should be causing a love that is to be brought from the husband to the wife.
The first active role that Reuven takes in Chumash is bringing the Dudaim to his mother. We are told twice that Leah stopped bearing (29:35; 30:9), and the second child of Bilhah is named in a way that insults Leah "I have fought with my sister and I was able [to be victorious]" (30:8). In addition, if we look at the fight between Rachel and Leah, it is clear that Rachel acknowledges that Leah is suffering from lack of love. It is specifically for that reason that Rachel allows Leah to sleep with Yaakov. Here it is clear that it is an issue of suffering which then brings Yaakov to love her. And it is Reuven that facilitates that love.
Here, as well, as we described above, Reuven is standing up for his mother's honor. The Gemara explains that Reuven understood that it was because of Rachel that his mother was not loved. However, after Rachel died he would not be able to accept that his mother's suffering comes about by way of a concubine. He therefore moved the bed, once again enabling his mother to be loved by Yaakov.
It was apparent to Reuven that because of his actions he had been separated from his father's love. It is for this reason that during the selling of Yosef, Reuven attempted to regain his father's favor by saving Yosef. However, Reuven was also looking for acceptance from his brothers, as they had lost respect for him after hearing about Reuven switching the beds. Both the above considerations made Reuven try to find a middle ground Yaakov's blessing to Reuven indicates that Yaakov did not approve of Reuven's methods. The characteristic of being "Pachaz Kamayim” – “unstable as water" (59:3) indicates that Reuven will intervene wherever he is able to, just as water flows to any spot that is open to it. This relates to us as Reuven did not know when it was appropriate for him to intervene and to what extent.
It is because of these events that Reuven as a tribe struggles to find acceptance, and they often finds themselves being afflicted. All the primary events of Reuven: his involvement in the Korach rebellion (Bamidbar 16), his request to settle Ever Leyarden (Bamidbar 32), and his building an altar as a way of connecting to Am Yisrael (Yehoshua 22) were all ways of looking to recover from having moved the beds, and now looking for a new way to gain recognition and love from Am Yisrael.
between his brothers and his father, and because a true compromise could not be reached with his brothers, he lost the opportunity to be loved by his father. However, we see that he tried to make it up by later offering his two sons as a collateral for Binyamin when Reuven took him down to Mitzrayim (42:37). All this suggests that after the incident of the moving of the bed, Reuven became more sensitive to his father's and his mother’s love, and did what he could to save his father from any suffering.