Aristotle argued that we become what we habitually do. If we spend our days thinking of everything that has gone poorly and how dark our future appears, we can think ourselves into misery.
While we should pay attention to the many injustices to be righted, we can also make the world a better place by being aware of the good things it already affords. We can change ourselves into the kind of people who seek out and celebrate things we can be thankful for.
One way to cultivate a disposition of gratitude is to give thanks regularly - at the beginning of the day, at meals, and at the end of the day.
Holidays, weeks, seasons, and years can be punctuated with thanks - grateful prayer, writing thank-you notes, and keeping a gratitude journal.