Because your Mommy did it for you.

My grandma used to cut it on both diagonals for me so I'd have tiny triangles. It was always better. ❤

Lust.

It all started when Archimedes found the sandwich he had lost, shouted eureka, and ran naked through the streets of Pompeii to tell Euclid of his discovery.

As luck would have it, Pythagoras was having lunch with Euclid that very day, and those two were often a sandwich short of a picnic.

Ruminating over Archimedes' sandwich, Euclid opined that if the sandwich was cut in twain both vertically and horizontally then the sum of the four squares would each the larger square.

Whereupon Pythagoras, eschewing Euclidean geometry, observed with a little strut and crust that cutting diagonally makes the sum of four triangles equal to the sum of four squares proving irrefutably and ineluctably that triangles are equal to squares.

Archimedes, who was eating the sandwich and not paying much attention by this stage, ran back down the street to escape the volcanic eruption and as a consequence completely misquoted a formula that they later attributed to Pythagoras.

I like it because I put peanut butter on it.

Definitely. No doubt. No argument here.