I joined because I'd been lurking for about a year and I already felt like I was a part of the community and had to keep reminding myself that I was just a guest, so as soon as I could, I did.
I wanted to join because I'm passionate about music, especially pop music, and it seems that "passionate" is a very good word to describe this community. It's a great way to find out about new music and keep updated, so I wanted to be a part of that. Also because all of the gifs and the dragging and shading was hilarious... but of course I won't be doing any of that.