When Moshe asked Yisro to stay on and travel with Bnei Yisrael to Eretz Yisrael, he said to his father-in-law, “V’hayisa lanu l’einayim.” The Targum explains that Moshe was telling Yisro that he had witnessed with his own eyes the tremendous nisim that Bnei Yisrael experienced in the desert. How could he walk away from that?
Later in the parsha when Bnei Yisrael complain about their lack of meat, the Torah tells us that Hashem was angry and “b’einei Moshe ra.” (11:10) The Chasam Sofer connects this phrase of “einei Moshe” with the previous comment of “v’hayisa lanu l’einayim” – the “einei Moshe” was Yisro! If Yisro, who joined Bnei Yisrael in the midbar, could be expected to come to emunah and trust Hashem based on what he had seen, surely the same could be expected of Bnei Yisrael who had also seen Yetzi’as Mitzrayim and kri’as Yam Suf! The “einei Moshe” acted here as a kitrug that caused the “ra,” caused the midas ha’din, to be further angered at Bnei Yisrael.
This, explains the Chasam Sofer, is what Miriam was complaining about when she said that the “isha kushis,” Moshe’s marriage to Tziporah, was the cause of problems. Tosfos writes that “kashim geirim l’Yisrael k’sapachas” because geirim are more careful in mitzvos than those who are FFB; the midas hadin asks why BN”Y do not rise to the same standard. Miriam thought that the presence of Yisro and Tziporah made Bnei Yisrael’s behavior look even worse in contrast.
(Parenthetically, the Ishbitzer connects this whole desire for meat back to the beginning of Braishis. It was only post-flood, post man’s downfall, that mankind acquired a heter to eat meat because it was only in that state (see Ishbitzer for the hesber) that the animal word would lend itself for tikun through achilah. Bnei Yisrael were on a spiritual high post-mattan Torah and post-Mishkan dedication, similar to Adam pre-cheit, but they wanted down from those heights and wanted back the post-fall tikun of eating meat. See also Sefas Emes in a number of pieces -- “Hisa’vu ta’avah,” (11:4) Bnei Yisrael wanted to be ba’alei ta’avah again, to be challenged by a yetzer ha’ra they could earn points by overcoming. This is reminiscent of the way a number of meforshim understand the sin of Adam.)
Putting aside the Chasam Sofer’s pilpul, the reaction of “b’eini Moshe ra” is not the response we are used to hearing from Moshe Rabeinu. Even after the cheit ha’eigel, Moshe appealed to Hashem for mercy – we don’t read that the cheit was “ra” in his eyes. Besides which, Moshe’s opinion is kind of besides the point once the Torah tells us that Hashem thought the complaints of Bnei Yisrael were unfounded and wrong.
Abarbanel suggests that Moshe’s intent was in fact to elicit G-d’s mercy. Moshe thought that if he displayed his own displeasure and disgust to the point of threatening to walk off the job, G-d would react by telling him that it’s not so bad and affirm the underlying goodness of Klal Yisrael. This time Moshe miscalculated – Hashem gave Moshe the support he claimed he needed in the form of a sanhedrin, and he still meted out punishment to Klal Yisrael.
What’s especially interesting is the order in which Hashem responded to Moshe: first, Hashem went through the process of appointing the 70 leaders to help Moshe, and only secondly did Hashem address the complaint of the people for meat. This whole story of the appointment of the sanhedrin seems to be a sidelight, yet the Torah not only juxtaposes it and links it hand in hand with the story of the people’s complaint , but makes it the primary concern of the parsha. Why?
You can read a great answer in this post at Beis Va’ad. I’ll share another great answer from R’ Avraham Shapira, the former Chief Rabbi and R”Y of Merkaz haRav: Rav Shapira explained that the people may have been complaining about meat, about gashmiyus, but the underlying problem was really a lacking in ruchniyus. Had the people been on the spiritual leve they should have been on, the type of complaints they were raising would not have bothered them. (So often people run after one pleasure after another, one bauble after another, a new house, a new car, another car, another gadget, and on and on, thinking that if only they had this material thing or that one they could finally be happy, but it’s not a lack of “things” that is really the cause of their complaints, but it’s a lack of any spiritual core to give them nachas ruach.) When a person is spiritually satiated, there is far less to complain about. Therefore, Hashem responded first with the appointment of a sanhedrin, with the appointment of assistants who could help Moshe reach the people and tend to their spiritual needs. That was the root caused of the problem; the complaints were just a symptom.