There is actually a lot of art that goes into programming a radio station. One of the radio formats that was out a few years ago had a slogan "We play what we want". They purposefully crafted the playlist to create "train wrecks", where one song was periodically followed by another that didn't feel like it belonged there.

The same thing with a live DJ -- their job is to watch the crowd, and the dance floor, and if the crowd looks like it is losing interest they alter the selection that makes everyone wake up and start moving. Or if it is getting too crowed, they put a song that has less appeal which causes a lot of people to leave the dance floor and head to the bar.