Reuvain's reaction

  וַיָּ֥שׇׁב אֶל־אֶחָ֖יו וַיֹּאמַ֑ר הַיֶּ֣לֶד אֵינֶ֔נּוּ וַאֲנִ֖י אָ֥נָה אֲנִי־בָֽא׃

Rashi explains:  אנה אני בא – אנה אברח מצערו של אבא.  

Reuvain comes back to his brothers, and discovers that Yosef is gone.  "If Yosef is gone," he exclaims, "How can I ever find refuge from our father's pain?"

(Interesting that Yosef here is called a  יֶּ֣לֶד.  Recall that at the beginning of the parsha we are told  וְה֣וּא נַ֗עַר.  The term יֶּ֣לֶד suggests the innocence of a child, someone who does not deserve to suffer for his actions.)

I think my son's favorite derasha in shas, if one can have such a thing, is the derasha on the extra vav in kabeid es avicha v'es imecha that comes to be marbeh achicha ha'gadol (he is the oldest).  (Minchas Chinuch discusses whether this is an independent chiyuv, or whether it is an extension of the din of kibud for a parent, i.e. that part of being mechabeid the parent is to also be mechabeid the achicha ha'hagadol.)  

The Alshich explains that Reuvain saw what happened to Yosef -- he was not sure if he was even alive at this point -- and thought that everything that had happened stemmed from the fact that Yosef did not show the proper respect for his brothers.  If so, thought Reuvain, if this is the punishment for failing to fulfill kibud for a brother, then what will befall me, as I failed to properly fulfill the ikar chiyuv of kibud av by being mibalbel Yaakov's bed.

A second pshat the Alshich suggests, which may explain why it is davka Reuvain who intervened to prevent Yosef's being killed, is that when Reuvain heard Yosef's dream about 11 brothers bowing to him, there was a silver lining to the story for him: he was 1 of those 11, he was part of the family.  His sin in being mibalbel his father's bed did not cause him to become a pariah. However, with Yosef gone, with those dreams dismissed as fantasy, Reuvain wondered if he truly was 1 of the 11, or perhaps that too was just fantasy.

The Midrash that teaches:

 והיכן הלך ראובן? ר׳ אליעזר אומר עסוק היה בשקו ובתעניתו, כלומר שהיה מתענה ולובש שקא על שבלבל יצועי אביו.

takes on a much deeper meaning in light of either approach.  It is Reuvain's sin against his father which is the backrop for his reaction to the loss of Yosef. 

Either way, I'm not sure what we are to make of the fact that the first thing Reuvain thinks of when he discovers Yosef missing is himself.  Notice (as Ibn Ezra does) the repetition of the word "ani" in the pasuk; it serves to highlight the ME of Reuvain's reaction.  Whether or not Yosef is still alive, whether or not Yosef's dreams have any relationship to reality, matters to Reuvain only in terms of what that means for his place in the family, or what it portends for the consequences of his sin against his father.  

R' Avraham ben haRambam has a far more charitable reading of the pasuk that reflects Reuvain's grief over the loss of his brother:  ואני אנה אני בא – צעקת ווי על חסרונו וגודל הצער על אבדונו