For all the "socialness" of the web, it's still very hard to convey emotions and feelings, anything beyond raw data. People make do and there are plenty of alternative forms of expression. Still, until computers can determine how we feel, YouTube is asking you to tell it.Based on your choice you get to enjoy videos that are "funny," "strange," "creative," "epic," "gross," "inspiring," and so on and so forth. The feature is called the "Moodwall" and it's now in testing.
It's not available to all users, but some are stumbling onto it. It seems that it's not working as good as it could all the time, but that's to be expected of experimental technology.
YouTube is always looking at ways of getting people to watch more videos. It's also trying to find ways of "understanding" what videos are about so it can make better recommendations.