Did you ever consider that you are a product? Your demographics are worth money, and thats one of the ways Google rakes it in. Every search you perform on Google is recorded in a database with your name on it. Read what the Google CEO thinks about your privacy: here and here
There is a good chance Google knows more about you than you do. Now, I am picking on Google, but almost all of them do this. Yes, I did say almost.
There is one search engine that will not collect and sell information about your web searches: Ixquick