(The mint, or cinnamon flavors are there, of course, to cover the fact that toothpaste tastes pretty bad. Those oils are amongst the strongest out there for flavoring.)
Reshared post from +Today I Found Out
The mint flavor of toothpaste isn't what makes it clash so bad with orange juice, it's actually the sodium lauryl sulfate and sodium lauryl ether sulfate, both of which are included in toothpaste to make the product foam and spread better so you feel like it is working better. The sodium lauryl sulfate interacts with your sweet taste receptors, making them less sensitive, and thus dulling the sweet flavor. In addition to that, it also destroys phospholipids in your mouth, which are compounds that have the same type of effect sodium lauryl sulfate has on sweet taste buds, except the phospholipids dampen your bitter taste buds.