Displaying nervous laughter at a serious situation can project you as an insensitive, rude, arrogant person and put you in an awkward situation. It is caused due to anxiety and stress and is your body's natural reaction to calm you down.
Why do I always smile in serious situations?
Nervous laughter is a physical reaction to stress, tension, confusion, or anxiety. ... People laugh when they need to project dignity and control during times of stress and anxiety. In these situations, people usually laugh in a subconscious attempt to reduce stress and calm down, however, it often works otherwise.
How do you force yourself to stop smiling?
Start by watching something you find somewhat funny but not too funny. Any time you smile, pause the funny material, stop yourself from laughing, and put on an expressionless face again. Once you have made it through a large portion without smiling, find something funnier and repeat the process.