Well, their interests are to make sure you see ads that are relevant to your interests, because advertisers are only happy if they actually sell whatever it is

Well, their interests are to make sure you see ads that are relevant to your interests, because advertisers are only happy if they actually sell whatever it is they're advertising. And on the face of it, seeing content that's relevant to your interests instead of content that's not seems like a good thing for users (completely ignoring for a moment all the scummy ads that prey on fear or ignorance to sell bad products). Speaking personally, if I have to see advertising, I'd rather see advertising that interests me instead of advertising that doesn't. However, I am not willing to sacrifice my privacy to do so.