Here's another metaphor. Your site and your users form a relationship. It might be a one night stand, a date, a partnering, an engagement, or a marriage. Perhaps a search engine or a link or an email set up the relationship. More and more is expected, and generally, less and less change is desired.
Stability plays a huge role in the relationship.
As a site grows in popularity and traffic, and the longer the site remains structurally unchanged, the greater the potential negative impact of a change to that site. Making Changes When people become familiar with a web site, it is very dangerous to make changes. Similarly, if you change your writing style, your graphics, or your navigation, expect your users to be frustrated and confused. I recently heard a great story about Dell's web site. The designers decided to make some changes to the site for one reason or another.