I love this tune! This slängpolska from Småland can go by many names, including Dansa Lilla Syster and Dansa Lilla Flicka.
It’s based on the song Dansen ungdom medan ni kan få, whose lyrics are all about dancing while you have your youth:
//: Dansen ungdom medan ni kan få innan era unga år försvinna (x2) ://
//: Ack det går så lustigt ja, det går så lätt (x3)
Innan era unga år försvinna (x2) ://
If you’d like to play it too, here is sheet music complete with suggested chords (just right-click to save and print the image):
Gilbert Music Studio 