A good spaghetti sauce will have chopped onion, celery and carrots, and minced garlic. Those will help sweeten it up. Roma tomatoes are usually the best to use, and I take the skins off and take the seeds out as well. If you don't have the other veggies in it, then it's just a tomato sauce, and not a spaghetti sauce.
You'll also want to add some beef stock or broth and red wine or even white wine... about a cup for each.
Also, a good extra virgin olive oil is good to use, but try to use it sparingly when sauteing the veggies because a cooked oil like that will change flavor, and you should add a little bit after you're done cooking. It will add a smooth, fruity flavor to the sauce.
Also, you want to make sure you have enough seasoning. I use dried oregano, salt and pepper, and then when it's done cooking, I add either a TON of fresh Italian parsley or Basil.
LAST, and THIS is probably what will sweeten your sauce up more than anything (other than sugar) would be to use tomato paste. Use about 1-2 tablespoons of it, and then add more if your still not satisfied.
Edit: Seriously? Don't add Mayo, and don't add a 1/2 cup of sugar... jesus.... it should be 1-2 tablespoons at most, or if any at all.
Oh, and to be honest, when I make my homemade sauce now, I just use canned tomatoes.... they are SO much easier to use, and actually give it better flavor.