Everytime they let you get nice and used to a good website, they go and radically change it, especially big names like Youtube, which really digs radical changes. The last BIG change was profile page changes which were majorly screwed up for a few months and still have a very big glitch that will allow you to do irrepairable & serious damage to someones comment section (I would tell you how, but its already well known).
Why do they do crap like this?
My theory is the same for YT, Microsoft, and any other big company - too many employees. To earn their paychecks, they have to be busy with something. In this case R&D and market research that says this & that (regardless if it holds true or not) must be done or people will get bored and migrate to other sites or products.
So in order to keep all the employees busy from R&D to technical to programming, they have to reinvent the wheel at least once a year. Otherwise come the layoffs and nobody wants that.
Sad, but true. :'(