"Parents always want their children to be happy, content and thankful to them for all their work and sacrifice. So when they see their children in a bad mood and unthankful, they're saddened. Its the same with our Heavenly Father. He has given us everything, but we are always unsatisfied and gloomy. Instead of thanking and praising God for everything, we only express our thankfulness with our lips; and our hearts remain cold. Joy is thankfulness; and when we are joyful, that is the best expression of thanks we can offer the Lord, Who delivers us from sorrow and sin."
(h/t Fr. Alexander Fecanin)-Elder Thaddeus