Feel joy with what the Almighty has given you, and do not focus your thoughts on what He has not given you.
Imagine how you would feel if you gave someone a present (although you had no obligation to do so), and he immediately complained that you didn’t give him twice as much! You would certainly regret having given him anything.
Failure to appreciate what the Almighty gives you is behaving in a similar manner.
Today, write a list of things you feel you have overlooked appreciating in the past. Be resolved to start feeling grateful for those things.
Sources: see Dubner Maggid; Sefer Hamidos: shaar hasinah, ch.2; Rabbi Zelig Pliskin’s Gateway to Happiness, p.37