Cicero once said, "Gratitude is not only the greatest of virtues, but the parent of all others". And that's really how important gratitude can be to living the happiest possible life.
That's because if you can be grateful for what's in your life right now - regardless of the circumstances you may be facing - then you have a positive baseline emotional perspective on life.
In fact, I would go so far as to say, if you can't be grateful for what you have now, then you may never be able to achieve true happiness, regardless of how much you get.
Both of these are effective ways to develop more gratitude. And if you can make the Three Great Things exercise a nightly habit, it will literally change your life.