"Why are we laughing?"/"I'm not sure."

That's the last line of this montage of moments from David Letterman's 30-years of TV. That's at Gawker, and I'm seeing it this morning, so I missed the tip that there was a 30-years celebration on the show last night. I didn't record it, and I don't keep a DVR subscription to the show, a show that I watched all the time 10 years ago. Or was it 15 years ago?