Let me see if I can use a metaphor that will bring the picture into focus. Please forgive discrepancies, as they are inherent in any metaphor.
If you, or someone you know, or someone you admire, is a fitness enthusiast/athlete, then you know that achieving a high level of fitness requires a tremendous amount of work and dedication. Workouts are made up of many different kinds of activities/increments/levels of intensity. They are not always easy, enjoyable, or pleasant. But the ultimate goal is an achievement of a level of fitness which is all the more wonderful for the effort that went into attaining it.
The Torah, and its mitzvot, are like spiritual exercise. They are our 'workout' in this life. They are not always easy -- either to understand or do; not always enjoyable. Sometimes we might find them unpleasant (fasting comes to mind). However, if we do them correctly, we can find a real energy, satisfaction, and exhilaration from the effort we exert. And the ultimate prize is that overwhelming accomplishment of achieving a closer relationship to G-d -- honoring, obeying, feeling close to Him, knowing we are on the right path.
Sometimes an athlete gets injured, or becomes lackluster in performance or workouts, etc. -- then the athlete starts rebuilding again, step by step, to reclaim his or her fitness. Sometimes we fall down the "ladder" of mitzvot -- we can't, don't, won't, or somehow just slide away from doing the right thing. That's okay, because there's always another opportunity to do 'teshuva' (make up for) and slowly climb back up that ladder again. That's why we have the mitzvot, so we have a concrete way to start that climb again, step by step by step.
Tomorrow, we fast and ask G-d for another year of opportunities to climb back up the ladder. Because, being human, all of us slide down and lose spiritual 'fitness' somewhere along the way. And so we recognize that and negate our physical needs for a day in order to concentrate on our spiritual quest to get closer to G-d.
ETA: In Judaism, the dichotomy between faith/belief and action is a false one, as has been emphasized before. The fact is, faith/belief is itself a Mitzvah -- an avoda (service to G-d). That is, although G-d bestows upon us the basic kernel of faith, it is up to us to 'exercise' and develop it. Sometimes it is challenged (as when bad things happen) and we must work harder to improve upon it, just like other mitzvot.
For those who are fasting: may you have an easy and meaningful fast. May G-d seal you in the Book of Life for the coming year. May you have a year of joy, peace, abundance, health, and only good tidings in your life. May you have a year of climbing the ladder of mitzvot successfully!