Personally, it's the tune that hits that part of the brain that sensual sex, fabulous food and Northampton Town scoring a 92nd minute winning goal does. Sometimes it may be an old favourite, it may also be a tune that is 'love at first hear', but whatever, you want to interperate that love, that spine tingling rush on hearing said tune by dropping everything and just dancing for the sheer joy of it all.
Of course, and happily, we all have different tunes that call us to the dancefloor.
So in answer to the question Miss Wasperella, it's the tune itself, although experiencing other people enjoying dancing to a tune can elevate it to a status it may not otherwise have enjoyed if just played at home.