Facebook Connect. Google was late to social. This annoyed them. They've been archiving data about you since you first used their search engine yet somehow, every login across the internet says "connect with Facebook." That means that the money flows into facebook's graph, not Google's. That means that the ad network is serving you up to facebook's advertisers, not Google's. That means those websites are monetizing you for facebook, not Google. So they're playing catchup. They're branding you with a universal identity so that they can sell your demographics no matter where you go. They know you've got lots of profile up on Youtube and they want you to agree (more importantly, they want proof that you agreed) to integrate your identity across all Google platforms so that they can present a better dataset to advertisers and make their offer more attractive. TL;DR - that brand on your hip? They want it to match the one on your forehead. They get more per pound at the slaughterhouse that way.