I think there is something about having pent up laughter, at a time we want to suppress it. It's almost like being unable to laugh out loud, due to the place or circumstance causes the laughter to be more difficult to contain, and last much longer than it otherwise would. I accompanied a person to a meeting recently and while walking down a hallway I noticed they had a pair of their wife's undergarment trailing from their pants leg cuff. I was irritated they had requested my assistance at the meeting, so I didn't tell them until they noticed I was laughing hysterically under my breath and they began looking for the source of my humor. I was fighting the tears from laughing under my breath through the meeting. Even hours later I would bust out laughing again till I cried.