Comparison is the thief of joy. Theodore Roosevelt
Well if this isn't the truth then I'm not sure what is. Getting caught in the comparison trap will steal the gratitude and joy from your life quicker than you can blink. If you find yourself complaining and lacking gratitude do a self-check. Are you comparing your life (finances, possessions, job, friends, family, etc) to someone else's? If so then you have your answer as to why the joy has been stolen from your life and heart. When we fall into the comparison trap we are communicating we aren't grateful for our lives. Joy is directly linked to being grateful and nothing steals joy like comparing ourselves to others and allowing envy to characterize our life.
Suggested Reading: One Thousand Gifts by Ann Voskamp
This book will guide you toward living a life of daily gratitude and being joyful.