Default is opt-in, not opt out
The short story is this. By watching what you click on in search results, Google can learn that you favor particular sites. For example, if you often search and click on links from Amazon that appear in Google’s results, over time, Google learns that you really like Amazon. In reaction, it gives Amazon a ranking boost. That means you start seeing more Amazon listings, perhaps for searches where Amazon wasn’t showing up before.
The results are custom tailored for each individual. For example, let’s say someone else prefers Barnes & Nobles. Over time, Google learns that person likes Barnes & Noble. They begin to see even more Barnes & Nobles listings, rather than Amazon ones.