Involuntary Emotions

There’s a reason it is difficult to sometimes suppress laughter or a smile. Facial expressions are closely linked to emotions. It is thought that the purpose of facial expressions is to convey emotions within a social group. Studies have shown that the link goes both ways, i.e., being happy can make you smile, and forcing yourself to smile can make you happier, which reinforces the idea that feeling emotions and exhibiting their signs are closely linked.

Emotions aren't entirely involuntary, but strong emotional reactions that are triggered by an external stimulus can be hard to override, i.e., controlling the physical manifestations of your emotional responses partly involves controlling the emotions themselves.

To add to this, emotional smiling and voluntary smiling (controlling facial muscles) are controlled by different centers in your brain. Researchers have observed that some people who have lost the ability to smile voluntarily because of a brain lesion can still smile due to an emotional stimulus that is not voluntary.