Why do they laugh in Mamma Mia when Donna says 'its like a trick of fate'?

Answer 1

Tanya and Rosie ponder the quiet bystanders, and Rosie mutters, "It's very Greek" Tanya and Rosie look at the Greeks then follow Donna into the house (Greek comedy).

Answer 2

I'd wondered that a lot and never found a response that answered the question. I was reading some old mythology and stumbled upon the Moirai who are the three goddess of fate. They weave the tapestry of fate for everyone, from birth to death, and also sing their songs with the Sirens. Since there are three of the ladies it is likely they are laughing because they are considering them comparable to the Moirai.