From The National Post, the main sources are Tim Harford and yours truly. Excerpt:

The answer, says Mr. Cowen, lies in the Alchian-Allen Theorem. Developed in 1964 by economists Armen Alchian and William R. Allen, the theorem states that adding a per unit charge to the price of two substitute goods increases the relative consumption of the higher price good.

In layman’s terms, "you don’t take a long trip unless you are going to make it worth your while," he says. Very few people in a long-distance relationship are going to fly across the country just to hang out in sweatpants with their sweetheart.