The saying says that "the grass is always greener on the other side", and yet there are a lot of circumstances in which people will fiercely defend their present circumstances, even when they're pretty clearly inferior to the alternatives. For example, most people show great devotion to their hometown, even when from an objective standpoint it may have little to recommend it. This isn't universal, but even people who have left their hometown often speak very highly of it.
So what makes some situations ones in which the grass tends to look greener elsewhere, while other situations elicit loyalty despite their faults? Does it depend on how much of an investment the situation demands for entry? A person who has invested years into a company might be reluctant to admit that another company is better. Yet, a person who invests thousands of dollars in a new car will often have second thoughts and wonder if he should have bought something else. Are there other factors? People may show more loyalty to situations that involve other people who have become their friends than to situations the relate only to inanimate objects.
It depends on your perceived happiness. If you're unhappy (i.e. hungry), then you will tend to believe that happiness is out there somewhere, waiting for you. But if you're happy, then you don't want to risk losing your current happiness for a chance at extra happiness.
Maybe it depends on the variability of evaluation criteria.
That is, in every domain, competing alternatives are evaluated according to certain criteria. If those criteria change easily and often, then a satisfying choice in one moment will seem less satisfying at a later moment, once the evaluation criteria for the relevant domain have changed. On the other hand, in domains where evaluation criteria are relatively fixed, preferences are less likely to change since the criteria by which alternatives are evaluated are less likely to change.
In the case of buying a car: fashions change all the time, as does the mood of the car buyer. If fashion and mood are factors in evaluating car preferences, then it makes sense that car preferences would be changing all the time too.
If judgments are based on fleeting criteria, they will scatter in a slight breeze. Foundations in sand vs. stone, and all that.
Our most inherent drive is for self-advancement; therefore, if we see something that might improve our quality of life in any way, we want to move towards it.
When people adamantly defend inferior circumstances, they may be feeling inadequate because of their circumstances. This is where people identify their self-worth with their surroundings. It's also a narrow-minded view of alternatives.
The whole philosophical light and caveman story comes into play here.
All of our most inherent drives are tempered by forces that, while not necessarily inherent or instinctive, are just as significant and often just as powerful. I think Ben Bateman has it right. While we may always have the drive to advance ourselves, we also have the ability to reasonably assess where we are currently and determine whether we are happy with it or not. It also becomes a judgement call on whether that "something else out there" is better enough to warrant the risk of unhappiness with the change.
There's also the problem of defining what "inferior circumstances" are with regards to the lives of other people. We can judge them as being inferior compared to *our* standards and what *we* want.. but we cannot authoritatively say that what *we* want is what *they* should want. The extremes of circumstances, of course, are exceptions to this.. as we can be fairly certain that few, if any, *want* to live in constant poverty or live in oppressive societies.. but in the scope of "the American experience", the judgement of the circumstances of others is a dubious endeavor.
I suspect that for the most part the grass looks greener to those who are dissatisfied with what they have, and once they have crossed the fence it will likely only be a matter of time before they will start looking over other fences.
The Bible admonishes us (Paul) to be satisfied with what we have.
A sage saying that you've likely heard. Don't be too excited about moving to a new place, because you will have to take you with you.
I think I like Nicholas' point that some shoices tend to change our preferences, while other choices are subject to preference changes from the outside. That makes sense and can explain why most people seem to like where they live (everyone around them is in the same boat, so there aren't many external influences to change their mind).