In response to their incessant complaining about the inferior quality of the manna the Jewish people were attacked by snakes, as the pasuk states, "the people spoke against Hashem and against Moshe, 'why have you brought us up out of Egypt to die in this desert for there is no bread and no water and we are disgusted with this rotten bread', Hashem sent against the people the venomous snakes and they bit the people and many died" (Bamidbar 21:5 - 6). Rashi explains that the Jewish people's slanderous speech about the manna warranted the punishment of snakes because the original nachash was the first one to speak slanderously when he falsely claimed that Adam and Chava were only prohibited from partaking of the Tree of Knowledge "for Hashem knows that on the day that you eat thereof, your eyes will be opened, and you will become like divine beings, knowing good and evil" (Breishis 3:5).
The relationship between lashon hara and snakes is confirmed by the pasuk, "one who breaks a fence, a snake shall bite him" (Kohelet 10, 8), referring to one who breaches the barriers of the teeth and the lips to spew negative speech about someone else. The libelous individual deserves to be bitten by a snake because the original snake first opened the door to lashon hara. For this reason, Yosef's life was also threatened by snakes. The pit where Yosef was cast by his brothers "was empty there was no water in it" (Breishis 37:24), but Rashi comments, "water indeed it did not contain but there were snakes and scorpions in it". The Kli Yakar submits that Yosef was trapped together with snakes as a punishment for speaking lashon hara about his brothers, as the Torah tells us, "and Yosef brought bad reports of them to their father" (Breishis 37:2).
However, Rav Pinchas Freidman (Shvilei Pinchas) suggests that the Jewish people were stricken with snakes after their disparaging comments about the manna for an additional reason. The Gemara (Yoma 76a) records that the students of Rabbi Shimon ben Yochai once asked him: Why did the manna come down every day in small portions instead of one large shipment that would sustain them for the entire year? He replied with the parable of a king who accepted to provide support for his son. The king knew that if he granted his son a substantial annual gift the son would only visit once a year when it was time to receive his allowance. Therefore, the king elected to give his son food in daily installments thereby forcing the son, who he loved and wanted to see, to visit every day. Similarly, Hashem chose to limit the Jewish people to daily helpings of manna instead of handing them one lump sum for the entire year, so that they would be compelled to interact with Him more often and daven every day for their needs.
The exercise of collecting the manna daily was intended to communicate Hashem's desire to have a relationship with the Jewish people and strengthen their awareness of Hashem's constant involvement in their lives. Perhaps this is alluded to in the language of the pasuk which describes Bnei Yisrael's initial reaction to the appearance of the manna when they declared "to one another 'man hu' - it is manna" (Shemos 16:15). Rav Yisroel of Modzhitz (Divrei Yisroel) notes that the letters of the words "man hu" can be rearranged to spell emunah - faith, because the purpose of the manna was to enrich and reinforce the emunah of the Jewish people in the providence of Hashem.
Rabbeinu Bachya (Parshas Chukas) claims that the primary grievance of the Jewish people regarding the manna was not in relation to its flavor or the manner in which it was digested, but rather to the size of the deliveries. They resented the need to trek out each and every day in order to gather their meager rations. Why couldn't they be given enough all at once to last the entire year so that collecting their panassah would not be a constant and daily struggle?
The answer to their complaint was provided by the emergence of the snakes, because the snake actually possesses an extreme version of the lifestyle the people requested. In the wake of the sin of Adam and Chava the snake was cursed, "and you shall eat dust all the days of your life" (Breishis 3, 14). On the surface the ability of the snake to consume dirt seems like a great blessing, since there is a constant and endless supply of food available. Indeed, the Gemara (Berachos 57a) relates that one who sees a snake in a dream is a sign that his livelihood is accessible to him, just as dust is readily accessible to a snake. What then was the nature of the snake's punishment? Rav Simcha Bunim of Peshischa explains that by giving the snake a perpetual stream of uninterrupted parnassah Hashem was in effect banishing the snake. Since the snake is not concerned about his livelihood, he has no reason to bother and interact with Hashem in the future. By striking the Jewish people with snakes, Hashem was contrasting the delivery schedule of the manna with the treatment of the snake. The work of gathering the manna every day, and the labor of earning a parnassah, while admittedly a nuisance and a bother, should ultimately be interpreted as a signal of Hashem's love for the Jewish people and His desire to continuously be involved in our daily lives.