This may seem rather simplistic and self-evident, but it really is key to understanding SEO. Google is a company. As a company they want to make money. As a matter of fact, they have a duty to their stockholders to do as well as possible.
Google has a number of different businesses. Many of these businesses have revenues in the millions of dollars. However, by far Google makes most of its money on advertisements. To make money on advertisements they need people looking at their pages.
They get people to look at their pages by providing quality search results for free. The better the search results the more people will look at their pages. If a user has an unpleasant experience with a particular search result they are a little bit less likely to use Google in the future.
Google strongly believes that efforts to artificially influence the search results make the search results worse. That is why they continually change their programs to screen out any websites that are found to be trying to influence the search results.
Any successful effort to manipulate search results is a problem for Google. Even if it results in better search results in a particular case, the fact that it works means that it could be used to cause the search results to be degraded.
Google’s updates its software frequently. Typically, it updates its software a bit more often than once a day. Most of these updates are minor, but some of the updates are huge, and take on a life of their own.
Google updates its software to produce better search results. Sometimes this is a general algorithmic improvement. At other times, the updates are in response to changing conditions on the Internet caused by people trying to fool Google and game their search results.
