Their parents were great friends, and, as such, they became best friends too. The fact that they were born only a week apart in the same hospital helped too. They had literally been friends since the instant they popped out of the womb.
As the years went by and the distinction between girls and boys became more obvious, their parents pushed more and more for a different relationship. No longer "When is Jennifer coming for Movie night," but more along the lines of "Why don't you take Jennifer out to a movie tonight?"
Ned and Moze were not entirely sure they hated it.