Youtube - I mean, Google, has decided all of their online services must be linked to a single account (you know, to make it easier for all of their benevolent users to access all of their datas). They had this silly project called Google+ that's been lingering around like the Blue Emac at your public library in the pool of the fancy high-tech social media websites. By forcing all of their other websites to require the Google+ account, they can now tell investors "Hey, people are using Google+! Look at all of the comments they are posting to YouTube videos and these Twitch streams! Isn't that glorious!"
My point is you will need a Google+ account (either link an existing one or create one, it takes a mere three seconds to do so) to get unrestricted access to your YouTube, Twitch, and other associated Google* accounts. You may also want to modify your privacy settings and sharing settings on your various website accounts, because (correct me if I am wrong) YouTube comments will be automatically posted to your Google+ "Wall", along with any other things you decide to post on a Google* website.
~Aayrl