Forsythe Pendleton “Jughead” Jones III from Archie Comics

First, many people are drawn to sweet, fatty, and rich foods. Cheese is fatty, tomato sauce is slightly sweet, and toppings can be very simple or complex. Pizza offers a complex taste, texture, and hits all parts of our palette with just one bite.

Second, cheese, tomatoes, and sausages all have a compound called glutamate which gets our brains highly excited. Glutamate makes our mouths salivate when it touches our tongue and makes us want more.

Third, there is the crispy versus soft and chewy factor. Humans desire that complex bite – guilty pleasures like Oreos, French fries, chocolate chip cookies, never get old. We crave the slightly crispy crunch of the outside and the soft and chewy inside. Pizza delivers that perfect combination. 

Also, the curled-up pepperoni edges and the cheese bubbles are the result of caramelization. The caramelization and browning at the top make the toppings rich and flavorful, the crust brown and crispy. This is called the “Maillard reaction” where the amino acids and sugars in high-protein foods heat and caramelize the foods.

If you wondered why pizza makes our mouths water, we say it's science.

Reference: https://theconversation.com/why-does-pizza-taste-so-good-125618