"Do not grumble against one another, brothers, so that you may not be judged; behold, the Judge is standing at the door."
There are so many ways that we can grumble. You can gripe and complain under your breath, to your best friend, to your mom, to a group, to a coworker -- even to yourself by dwelling on a "situation" longer than you ought. Grumbling comes naturally, but it reflects an ungrateful heart. We have so much to be thankful for! Grumbling doesn't help anyone, but a thankful heart encourages, uplifts, and gives glory to God.
One Thing: Be thankful for the people in your life and focus on their good qualities instead of complaining about them.