Frankly, it's the fantasy element. A woman could be walking down the road, dealing with a knotty emotional issue, and suddenly the people around her are singing and dancing with her about that issue that's only in her head. Everyone knows the same song and dances to the same beat, in sequence. That trope never gets old to me. I'm like a toddler who likes playing that same game over and over again.
Music is good, dancing is good. But just the idea of a musical is what brings joy to me.