In my opinion, there are two key components to a mouthwatering marinara sauce: (1) Sofrito; and (2) Parmesian Reggiano rinds.
For the Sofrito, I use 2 carrots, 2 celery stalks, an onion and a few garlic cloves. I gently pulse the first three ingredients in a food processor until chopped (not too ground down). Then I sautee' them in a dutch oven until tender (approximately 10 minutes). Last, I add the garlic for 1 minute as I do not want it to burn. Next, I add a can of San Marzano (yes, the type of tomato is important) tomatoes. San Marzano represents a region in Italy and they are considered to be the premier plum tomato option for Italian cuisine. For an added touch of elegance, add a can of Italian cherry tomatoes (pomodorini) to the dutch oven. They are plump and sweet and rich and delicious! The pièce de ré·sis·tance is adding a couple of Parmigiano-Reggiano rinds to the luscious mixture. The cheese from the rinds will melt into the sauce and add incredible flavor and texture.
Now, as I often say, it is about patience. Simmer the sauce for hours. I typically let it marry all of the flavors together for at least six hours. If it becomes too think, adding water is just fine. Enjoy!