Why Did cavemen dance?

This begs two questions; were there actually cavemen? And what makes you think they danced?

Starting with the first part; caveman is something of a 19th, early 20th century stereotype. Few scientists working in the field would refer to "cavemen", mainly because there isn't much evidence that early humans actually lived in caves.

Going to the second part of the question; why did they dance? It's very hard to speculate about why prehistoric hominids did things. It would be hard enough to come up with a good answer for modern humans, let alone prehistoric ones.

However, I would postulate that humans are natural pattern detectors - it can be argued that a definition of intelligence is "the ability to detect patterns" and early hominids, who we know made stone tools, would have heard the repetative sounds of the toolmakers and, presumably, moved along with the sounds.