We don’t think of it as a “gross sin” when we complain, do we? When I read this, I was a little convicted. I’m not one who complains a lot, but I do complain - and the truth is, I am blessed! We learn from David that the way to God’s heart is to pour it out to God and not to others - this is the healthy way to get it out of your system and then trust God with the outcome.
I don’t think God wants us to be happy about worldly circumstances, He just wants us to love others and trust Him above all in spite of the situations. “Do everything without complaining and arguing,” Philippians 2:14 NLT