YouTube is gently prodding users to use their real name when you comment or upload a video. If you decline, Google makes you select a valid reason, like “My channel name is well-known”. Google is doing this to fight against strange, rude and offensive comments. This change will improve the quality of the comments you will find on YouTube.
From Google Blog:
Maybe you’ve outgrown that username cutepuppies99, which seemed like such a good idea at the time. Or perhaps your friends are missing out on your mad origami skills, because they don’t know you are the genius behind origamiboy1981.
Starting today we’re giving you the ability to change how you appear on YouTube, with the option to use your Google+ profile on your YouTube channel. One Google-wide identity was something that proved popular with new YouTube users when we began offering it in March, so we are now extending it to existing users.