Cracking Up

"Cracking up" has two meanings. It can mean laughter or insanity. What is the relationship between these 2? If you are cracked, you are nuts or lacking common sense.
If you crack a smile, it's a spontaneous movement you can't suppress, and cracking up is indulging in hearty laughter that can't be denied.
But cracking under pressure is not good.
Cracking up is not good, even if it is laughter, because it is often used for inappropriate laughter. If you crack up in a church or in class, you get in trouble. And if you crack up laughing repeatedly, people will think you are really cracking up (going crazy).

Crack up also means a crash. So, here's hoping all your crack-ups are the laughing kind!