People who have a generally negative view of the world have a hard time enjoying life. If most people are out to get you, if things probably won't work out, if you are stuck in a situation you can't do anything about...the life goes out of your eyes.
On the other hand, if you feel you have been incredibly blessed, if you believe the best about others, if you can see a way out of your troubles...there's a sparkle in your eyes that reveals life within.
It's not to say your life has to be going very well for you to have a hope-filled heart. In fact, many people I know who have had very difficult lives and who are in very difficult times are shining examples of hope, joy and inner vibrancy. And vice versa, many people who have very 'blessed' lives, who have relatively few troubles, are dead within, hopeless in a dark world.
Learning to manage your outlook in such a way that you keep it authentically renewed and positive goes a long way towards having a healthy, fully alive and functioning heart.