Why it's funnier when you're not allowed to laugh

I don't think I've ever laughed harder than during a church service, when something faintly ridiculous caught my eye. My friend saw it too, and once she started laughing, it became impossible to stop. Years later I've tried to explain what was so hilarious, but it seems you had to be there. What was it about the combination of the situation—sometimes referred to as "church giggles"—and shared laughter that made it so funny?