"Folk song and folk dance are interrelated with each other" explain it.

Explanation:

Dance is performed to the music, and therefore has to be contemporary with it;  for example, happy, energetic songs follow the bouncy dance. The same goes for folk music and dance.

However, folk songs and dances originate from the same people and the same occurrences and intervene with each other. Folks who are generally more upbeat, usually have more energetic dance and music (Spanish or Irish). At the same time, those who are considered to be more "laid back" more likely perform easy-going dance and music (for example northern greek and Montenegrin dances). Folk dances reflect the history and the story of these people, as well as the music and lyrics of the song. They are all interrelated with each other. In the same manner, the musical instruments which are used to perform the songs are those most common for those particular people. Dance techniques are adapted to those exact tempos and sounds which musical instruments can make.

Therefore, if you want to know the folk dance fully, you have to understand the music and culture as well. This is why it is also vital to know the full history of music, dance, and culture when getting into the folk dance. For example, when Irish dance instructors are doing exams to become professional trainers, they have to also show that they know Lilting, traditional singing, common in Gaelic parts of Ireland.