For being more precise:
Tomatos are acid! This is the characteristic of tomatos. And this is also the reason of tomato success.
Thanks to this quality tomato makes edible all other acidless food.
Acidity produces a strong salivation that makes easiest the first approach with food you eat, clean palate and give the wish to eat again at once.
But also acidity makes bite fairly bound in mouth up to bottom.
In fact before tomato, acidity was taken from citrus and fruits.
Of course acidity makes food bitter. This is the only reason of bitterness of tomato sauce.
There are two main ways we use in Italy to remove that bitterness:
- a pinch of sugar in the sauce while
cooking it. This is the main and
- a pinch of bicarbonate of soda.
The garlic and also triturated carrot are other ways but alter the sauce. I don't advice those for genuine tomato sauce.
I hope you have a better understanding of bitterness and I could help you validating what Bruce said about how to remove it.
The Italian cooker ;)