It's acceptable for meetings to have some obligatory humor but spontaneous or sustained laughter is inappropriate. Productive people don't laugh at work. Frankly, it can make you serious just trying to do laughter the right way. It's hard to say when we got so serious about controlling laughter in our society. We are increasingly controlled and controlling, and let's face it.
Laughter is out of control.
Your muscles relax, and you could appear klutzy, spit out your food, or wet your pants. Your face contorts, you gasp for air, and can't talk. You hold your belly while you make unintelligible laughter sounds. Dignified and professional is not the image you portray at that moment in time. Although we pursue pleasure avidly today, we don't allow ourselves to feel it when we have it.