Why is the distance between two meridians at the North Pole 0 miles?

They are meeting at the North Pole. the North Pole & South Pole are the only places where lines of longitude meet and the distance is 0 miles.

Explanation:

Unlike parallels, which are always parallel to each other, meridians converge as they approach the poles. This causes a degree of longitude to vary from approximately 69 statute miles at the Equator (a great circle) to 0 miles at the poles.