As users browse websites in the AdSense network, including YouTube, Google stores an advertising cookie in a user’s browser to understand the types of pages that user is visiting. This information is used to show ads based on interest categories that might appeal to users. For example, if a user browses many sports-related websites displaying AdSense ads or watches sports-related videos on YouTube, Google may associate a sports interest category with their cookie and show the user more sports-related ads. Users can also edit their list of interest categories within the Ads Preferences Manager.
Users can opt out from the interest base advertising by downloading a cookie to their browser, however since cookies are temporary you’ll need to download the cookie again and agin every time you clean your browser cookies. Fortunately Google also provides a browser plugin which will let you opt out permanently from this kind of tracking.