You laugh at two situations during a serious matter:
When somebody cracks a lame joke, which at that moment sounds amusing because the whole world is serious around you.
When you crack a joke and people around you chuckle.
Sometimes this gets really annoying to some people in such situations, especially if anybody is actually is leading to scenario or is very rational about it. Then in that case that would be very impolite and ignorant. People think what they like to think, if they think the scene is serious, however it might never be so or is nothing but just a stupid drama whatever, then you might as well just play along without complaining ..or laughing.
