למה העליתנו ממצרים, למות במדבר. כי אין לחם ואין מים ונפשנו קצה, בלחם הקלקל
Why have you brought us to die in the wilderness? Because there is no bread, and because there is no water. And our souls are disgusted by this rotten bread. -Numbers 21:5
Rashi thought that the issue was that the Israelites falsely attributed G-d's power to Moses. I'm not going to necessarily disagree with that, but I'm going to see if I can find something a little more gratifying. Maybe it was a lack of faith, but this wouldn't be the first time that the Israelites expressed their doubts. Let's keep in mind what faith is, which is a strong belief or trust in something or someone, often without any proof. For this generation, they had many forms of proof. They had witnessed the Ten Plagues, the splitting of the Red Sea, and the pillar of fire. They had all the proof they needed.
In my humble opinion, this was not an issue of doubt in G-d, but a particularly bad case of ingratitude. How so? The Israelites had a well of water that constantly followed them in the desert. They also had a daily portion of manna fall from the sky, which is a substance that can be made to taste like whatever one desired. How can they kvetch about something they already had? One is that when you put your desires into the physical realm, you will never be satiated. The Talmud is known for saying "He who has 200, wants 400." We often make the same mistake that the Israelites made, which is that we are often unthankful for what we have. In Hebrew, the term for gratitude is הכרת הטוב, which literally means "recognizing the good." Not only did the Israelites not recognize the good they had, but they went as far as distorting as "rotten bread."
Whether the שרפים were literally serpents or whether this was a literary device to make a point, we can learn three things from this passage. One, the good we have in our lives exists. It's right in front of our faces, much like the manna and water were for the Israelites. Although there are undoubtedly times that are bad, whether you're literally wandering in a desert or are in a dark place in your life, the good is always there, even if it is difficult to see. Second, we must not become indifferent to the good we already have. Part of the challenge of gratitude is that we become so accustomed to the things and people we have in our lives. The Israelites were so used to having the manna and water that they took those essentials for granted. When you realize what you have in life, don't take it for granted because you might not have it one day, or in the case of the Israelites, G-d might send some שרפים your way. Finally, upon realizing what you have, you go in the opposite direction of the Israelites' kvetching by expressing gratitude for what you have. Being able to express gratitude is living up to name Jew (the Hebrew being יהודי), which comes from the root להודות ("to thank"). For some of us, to be thankful in every situation might be too much. We should still work on expressing it as often as we can. If you want a basic, minimalist place to start, start with this Yiddish proverb: "If you can't be thankful for what you have, be thankful for what you have been spared."