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.
The result is overblown expectations ("are we having fun now?") and excess pressure on the relationship. Here is a previous MR post on this topic.