If you want to save the world, be happy; in fact the Rebbe himself said “Happiness will bring Moshiach!” – besides for the simple benefit, that happiness sure beats misery (though we seem to be addicted to misery, as if it holds some hidden cure to our ailments, for our ego cares nothing about our happiness, only its low self-esteem) additionally, when we are miserable, we become jealous of people who are happy, and jealousy over happiness is the cause of all the evil that ever existed!