The theory which explains that taking two goods things and putting them together creates something better.
Reasonable Person asks: Is peanut butter good on its own? Is jelly good on its own? Then I am sure that they'll go great together in a sandwich. That's simply the Peanut Butter and Jelly Theory.
Dude 1: Ew, dude! Are you eating cinnamon fried apples with steak?
Dude 2: Are cinnamon fried apples good on their own?
Dude 1: Well, yeah.
Dude 2: Is steak good on its own?
Dude 1: Of course!
Dude 2: Then there you have it! It's basic Peanut Butter and Jelly Theory.
(As you can tell, this theory does have quite a few exceptions)