In the letter in which the key word is "joy" or "rejoice", the apostle Paul writes: "Do all things without grumbling or questioning" (Philippians 2:14).
What is the difference between grumbling and constructive criticism? The same as the difference between flattery and compliments. When you flatter, you do so because you want something in return. Flattery is giving compliments from selfish motives. Compliments are given in the best interest of the other person.
Constructive criticism is given in the best interest of the other person. It is given to help the other person be better. Grumbling and complaining is done out of selfish motives. I complain because you are not fulfilling my expectations. While constructive criticism is helpful and beneficial, grumbling and complaining is sinful.
The classic biblical example of grumbling and complaining is Numbers 11. In the wilderness in Exodus 16, the Israelites were complaining about what God had given them to eat. They did not appreciate His blessings. So they grumbled. God responded by giving them quail and bread. Meat and potatoes. What more does a man want?
But in Numbers 11, the Israelites had grown tired of God's blessings and grumbled about the quail and manna. "And the people rose all that day and all night and all the next day, and gathered the quail. Those who gathered least gathered ten homers. And they spread them out for themselves all around the camp. While the meat was yet between their teeth, before it was consumed, the anger of the LORD was kindled against the people, and the LORD struck down the people with a very great plague" (11:32-33).
God does not appreciate grumbling and complaining. The next time you feel like saying something negative, ask yourself - Do I want to say this to help that person be better or is it motivated out of selfishness?