Why might our economic goals change over time?

Because our needs change.

In youth we might want our lawn cutting money to buy roller blades or bicycles.

In our late teens and early twenties might call upon our money to go to college.

In our late twenties, we might want to start a family or take on a mate.

In our thirties and 40s we may want to solve the problem of housing and our children's education.

In our 60s we turn our attention to either health problems or downsizing or setting up trusts for our children.

Things happen. Our goals change in response to what happens to us. Our economy changes with our goals.

It's a cycle.

It's a simile describing the apperance of the morning fog.