Rebuking the impetuous
ראובן בכורי אתה כחי וראשית אוני יתר שאת ויתר עז: פחז כמים אל-תותר כי עלית משכבי אביך אז חללת יצועי עלה
Reuven, you are my firstborn, my strength, the first of my vigor. [Potentially] exceeding in position and exceeding in might. Hasty as water, you will not exceed, since you went up on your father’s bed. Then you profaned that which went upon my couch
As Yaakov’s life was ending, he took the opportunity to give his children their final blessings. While accenting their unique traits, he also informed them of their shortcomings. He started with his firstborn Reuven by rebuking him for an incident that had happened decades earlier. When Yaakov’s wife Rochel died, he moved his bed into Rochel’s maidservant Bilhah’s tent. Reuven felt this was an affront to his mother Leah, who should have become Yaakov’s primary wife. Reuven audaciously moved his father’s bed out of Bilhah’s tent and put it into Leah’s. At the time, Yaakov said nothing. Now that Yaakov’s life was ending, it was now or never to rebuke Reuven.
Yaakov informed Reuven that he had the potential to have two positions over his brothers: the kingship and the priesthood. Originally, all Jewish Kings were to be descendants of Reuven, as well as all the Kohanim. Due to his sin, he lost both positions. Yaakov informed him of the reason for his demotion. Besides for the impetuous behavior, his actions profaned the Shechinah, the Divine Presence, which regularly resided above his bed. We can understand the impropriety of Reuven meddling with Yaakov’s married life, but is there a deeper connection between his sin and the resulting punishments of losing the priesthood and kingship?
Rashi clarifies that Reuven was guilty of two sins. His rashness to move his father’s bed was out of anger for what he felt was an insult to his mother. Anger is never justified. Further, by moving the bed, it showed a lack of sensitivity to sanctity. He acted like this was any regular bed, which is fit to be moved at whim. He didn’t pay heed to the fact that the Divine Presence rested on Yaakov’s bed. This made it a holy object, similar to the vessels in the Holy Temple. His lack of respect was a desecration.
These two sins showed he was unworthy of the kingship and the priesthood. We are taught that anyone who shows anger is demoted from leadership. This is learned from Aliav, who was chosen by Hashem to become king. However, since he displayed anger towards King David, this potential position was withheld. Reuven, by displaying his anger towards his father, showed that he was unworthy to have descendants who would be King.
By profaning his father’s bed, it became obvious that the priesthood was inappropriate for Reuven’s descendants. As the Jewish people’s emissaries in the Holy Temple, they had to be extra sensitive to sanctity. This could also be the reason that the firstborn status was taken from Reuven and given to Yosef. Before the Mishkan was built, offerings were handled by the firstborns. That task was eventually handed over to just the Kohanim. Nevertheless, we see that being a firstborn also carries with it a necessary sensitivity to holiness. Since he was lacking this, he also lost his firstborn status.
 Based on Be’er Yosef to Genesis 49:3-4
 See below that this is how this the verse is understood by the commentaries
 Genesis loc. cit.
 Described more harshly in ibid 35:22
 Rashi ad. loc., quoting Shabbos 55b
 Inter alia, Rashi, Targum Onkelos, and Targum “Yonasan” to Genesis 49:3. Rashi says יתר שאת alludes to נשיאת כפים, the priestly blessing. יתר עז alludes to kingship, as seen from the verse ויצר עז למלכו (I Samuel 2:10)
 Targum Onkelos and Targum “Yonasan” loc. cit. also point out that Reuven lost the firstborn status. This comes from Berachos 7b and Bava Basra 123a, based on I Chronicles 5:1. Rashi doesn’t bring this, but the Be’er Yosef addresses it at the end
 See Sotah 17a, איש ואשה זכו שכינה ביניהם, with Rashi
 To Genesis 49:4
 Mishneh Torah Hilchos Deos 2:3
 Pesachim 66b
 See note 7
 See Rashi to Genesis 25:31 and his source Bereishis Rabbah 63:13