There's a phenomenon at cocktail parties where the person you're talking to darts his eyes around the room in search of someone more interesting, powerful, or famous. The guy is asking himself the question: Can I do better?
This is the same question that lurks in the minds of people in medium to long-term romantic relationships: Can I do better? Am I settling? Could I possibly date or marry someone more attractive, more intelligent, and more compatible overall?
The "grass is always greener on the other side" is the idea that we glorify what we don't have or can't see. When trapped in a long-term commitment, we overestimate the indeterminable opportunity costs.
I believe that in romantic relationships where neither party has significant relationship experience outside their current one, this can-I-do-better question is the most frequent ultimate cause for break-up. (The stated proximate cause usually differs.)
I wonder if the strongest, longest relationships are those that do have an extended break along the way. With a break, both parties can compare being in the relationship to being single or (preferably) to being with someone else. If you do get back together, you have a sense of whether you can in fact do better, or whether what you have going is as good as it gets.
The majority of couples that break up do not get back together; but my theory is that those that do are stronger in the long-run than those which never have the break in the first place.
Port this theory to the professional world. Say you're running GE's leadership development program. Say you hire a top-notch recent college grad, he spends 5-6 years working his way up the ranks, and you're grooming him for a future senior management position. If his career experience with other companies has been minimal, should you send him to work for a totally different organization for two years, and then hire him back at GE, to satisfy his can-I-do-better itch? Should non-competitive companies, with low turnover, in different but related industries, have exchange programs?
Bottom Line: Wondering whether there are brighter pastures on the other side is a source of stress and dissatisfaction for people in long-term commitments. They start scanning the horizon for something better. The commitment is therefore strongest when this curiosity has been satisfied, and they return to the commitment with a better understanding of their real (versus imagined) opportunity costs.