It’s the canonical example of reference-dependent happiness. Someone from the Midwest imagines how much happier he would be in California but when he finally has the chance to move there he finds that he is just as miserable as he was before.
But can it be explained by a simple selection effect? Suppose that everyone who lives in the Midwest gets a noisy but unbiased signal of how happy they would be in California. Some overestimate how happy they would be and some underestimate it. Then they get random opportunities to move. Who is going to take that opportunity? Those who overestimate how happy they will be. And so when they arrive they are disappointed.
It also explains why people who are forced to leave California, say for job-related reasons, are pleasantly surprised at how happy they can be in the Midwest. Since they hadn’t moved voluntarily already, its likely that they underestimated how happy they would be.