In my opinion, laughter really is the best medicine.  Like a steam bath, it opens up all your pores and lets the bad stuff roll out.  That's why I use humor on the air - it's a diagnostic which makes your body, psyche, and soul all feel better.

Laughter seems to have an evolutionary benefit.  Laughter is a feature that we share with other great apes such as the chimpanzee and gorilla, which suggests that it is an ancient behavior.  People of all ages and cultures laugh spontaneously, and they spend quite a bit of time doing it.  Interestingly enough, if you ask most women what traits they want in a man, a sense of humor is usually first or second on the list.

Laughter also operates as a social connector.   Groups are important for human survival, and across evolutionary time, groups got larger and socially more complex, which raises the interesting question about how these groups could be held together. Other primates groom each other to smoothen social interactions, but this is impossible when groups get really large. One solution to this problem is laughter. Through laughing, we can quickly establish a good relationship with each other, and because it is so contagious, it can quickly spread through a crowd.   For example, if you're watching a movie with other people and someone laughs, there is an instant connection. 

In addition, laughter helps facilitate your capacity to learn new things.  When somebody teaches you something with humor, you usually retain the information better.  That's why, for example, kids learn faster and better through play-learning. 

Lastly, laughter helps alleviate pain.   When you laugh, endorphins are released in the brain and act as a kind of legal drug inside your head.  According to studies conducted by researchers from Oxford and VU Amsterdam, being exposed to comedy can raise your pain tolerance as much as 50 percent.  I believe the same is true for emotional pain as well.  Humor opens up people to hear things that they are often uncomfortable hearing.  There is even some research on patients which shows that exposure to humor and comedy helps them reduce their medication intake.

Without humor, life would be quite dreary.  Laughter works in the same way as a good massage or an intense jog (but without all the stress on the knees).  It's relaxing, social, and there are no side effects other than the occasional bad joke.