Frog and Toad. Elephant and Piggie. George and Martha. Bink and Gollie.
There are pairs everywhere in literature, but it seems like friendship gets a starring role in books for early readers. Frodo and Batman and Anne of Green Gables may have sidekicks, but Frog, Elephant, George, and Bink have partners. (Don't those names sound strange on their own?) The "and" is just as important as the names sandwiching it.
Having two equally billed characters makes sense for some practical reasons. It allows for humorous dialogue and simple conflict in stories that don't need to be complicated enough for friendship to be an afterthought. When you're just learning to read, you've got enough to wrap your head around without Gollie fighting dragons and then telling her wisecracking pal Bink about it. Friendship itself, if portrayed entertainingly, is story enough.
In fact, when you're five, six, or seven, "best friends" are starting to enter the picture. At that age, the idea of sharing top billing with a friend might be just as exciting as having it for yourself.