A general theory that explains laughter is called the relief theory. Sigmund Freud summarized it in his theory that laughter releases tension and "psychic energy". This theory is one of the justifications of the beliefs that laughter is beneficial for one's health. This theory explains why laughter can be as a coping mechanism for when one is upset, angry or sad.
Philosopher John Morreall theorizes that human laughter may have its biological origins as a kind of shared expression of relief at the passing of danger. Friedrich Nietzsche, by contrast, suggested laughter to be a reaction to the sense of existential loneliness and mortality that only humans feel.
For example, this is how this theory works in the case of humor: a joke creates an inconsistency, the sentence appears to be not relevant, and we automatically try to understand what the sentence says, supposes, doesn't say, and implies; if we are successful in solving this 'cognitiveriddle', and we find out what is hidden within the sentence, and what is the underlying thought, and we bring foreground what was in the background, and we realize that the surprise wasn't dangerous, we eventually laugh with relief. Otherwise, if the inconsistency is not resolved, there is no laugh, as Mack Sennett pointed out: "when the audience is confused, it doesn't laugh" (this is the one of the basic laws of a comedian, called "exactness"). It is important to note that the inconsistency may be resolved, and there may still be no laugh. Due to the fact that laughter is a social mechanism, we may not feel like we are in danger, however, the physical act of laughing may not take place. In addition, the extent of the inconsistency (timing, rhythm, etc.) has to do with the amount of danger we feel, and thus how hard or long we laugh. This explanation is also confirmed by modern neurophysiology (see section Laughter and the brain).
Laughter can also be brought on by tickling. Although it is found unpleasant by most people, being tickled often causes heavy laughter which is thought to be a reflex of the body, and is often uncontrollable.
It has also been shown that laughter is highly contagious, as shown in one case where schools in an area were closed for six months as a result of a massive laughter epidemic.