I would say “practicing gratitude” to convey the thought of focusing on what one has in life and ruminating on little things in a positive way, the glass half full. What would be the opposite word or phrase for when one habitually focuses on what they do not have. Not necessarily in relation to someone else as in envy or jealousy.
As suggested in the comments, here’s some clarification:
Ingratitude just implies a lack of gratitude, I think. I’m looking for the other side of the coin. Someone who would practice gratitude might take note of the small things in life, even keep a gratitude journal, or look for ways to express such to a friend. But I’m looking for a word that describes someone who focuses on what’s missing, as if the person is almost making a list of what he doesn’t have and complaining or ruminating over that.