It's well-known that the Teenage Mutant Ninja Turtles love pizza, especially in the movies, but that wasn't always true of the characters. Surprisingly enough, the Teenage Mutant Ninja Turtles pop culture phenomenon has now been going (mostly) strong for well over 30 years at this point. Not every project starring the group has been a smash hit, but they never seem to be too long a stretch without the crime-fighting reptiles managing to convert yet another new generation of devoted fans.
The fact that the Teenage Mutant Ninja Turtles like chowing down on a slice of pizza or ten has long since become a cliche, with the image of Leonardo, Donatello, Michelangelo, and Raphael with pizza in their hands being one of the first people call up when thinking about them. To be fair to the group, most people tend to love pizza, and it's arguably one of the most versatile foods as far as when people might eat it. Some might eat pizza for breakfast, others for lunch, and still others for dinner, or even just have a slice for a snack.
Still, real turtles don't enjoy pizza, as the animals can't digest dairy, ruling out a cheese-covered food. So, why do the teenage mutant variety love it so much? For a few reasons.
What some fans may not realize is that the Teenage Mutant Ninja Turtles weren't obsessed with pizza in the original comic books. That aspect of their characters wasn't introduced until the 1987 cartoon series, which made them household names. It was retained for the 1990 movie and its sequel, and every subsequent version in movies and on TV. While turtles can't eat pizza, the pizza love was created to reflect their teenage side, as there's no food more closely associated with teenagers than pizza.
Additionally, it increases the Teenage Mutant Ninja Turtles' association with their home city of New York. There are few things more identified with New York City than its pizza, making the turtles' love of it into a reflection of their status as New Yorkers. Incidentally, NYC was chosen to be the Turtles' home by creators Kevin Eastman and Peter Laird as part of their original intent for the property, that being a parody of the often overly serious superhero comics of the time. New York City was heavily overused as a locale and was chosen for that reason. By the time the movies started releasing though, the Turtles had become a positive symbol for the city.