Rebbe Nachman says that those without emuna (faith in Hashem) have no life, even in this world. As soon as things go against them, they are left with nothing. They think everything is determined by nature and have no place to turn. When trouble strikes, they are left without any source of hope.
A person with emuna believes in Hashem and has a very good life. When trouble strikes, this person’s emuna still inspires him. This person trusts in Hashem and knows that everything is for the best. If one must suffer, the person realizes that it will atone for sins committed, or that it will ultimately bring a much greater benefit. No matter what happens, this person realizes that Hashem does only good. Therefore the person of emuna always has a good life, both in this world and the next. (Sichot HaRan #102)
Above all else, we should completely trust in Hashem. Trust (bitachon) is the ultimate measure of the depth of a relationship with anyone, and the extent to which we are prepared to trust someone is the extent to which we are one with that person, or with Hashem for that matter. And, since we should have as close a relationship with Hashem as we possibly can, then we are meant to trust in Hashem completely.
Adapted from Be Positive, by Pinchas Winston