Laughter

Laughter is an audible expression (written as ha ha ha or lol etc.), or appearance of merriment or happiness, or an inward feeling of joy and pleasure (laughing on the inside). It may ensue (as a physiological reaction) from jokes, tickling, and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter. Strong laughter can sometimes bring an onset of tears or even moderate muscular pain. Laughter is a part of human behaviour regulated by the brain. It helps humans clarify their intentions in social interaction and provides an emotional context to conversations. Laughter is used as a signal for being part of a group  it signals acceptance and positive interactions with others.
Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback. An extreme case of this is the Tanganyika laughter epidemic. This may account in part for the popularity of laugh tracks in situation comedy television shows

Scientific explanation:
Modern neurophysiology states that laughter is linked with the activation of the ventromedial prefrontal cortex, which produces endorphins after a rewarding activity.

Research has shown that parts of the limbic system are involved in laughter The limbic system is a primitive part of the brain that is involved in emotions and helps us with basic functions necessary for survival. Two structures in the limbic system are involved in producing laughter: the amygdala and the hippocampus. The December 7, 1984 Journal of the American Medical Association describes the neurological causes of laughter as follows:
“Although there is no known ‘laugh center’ in the brain, its neural mechanism has been the subject of much, albeit inconclusive, speculation. It is evident that its expression depends on neural paths arising in close association with the telencephalic and diencephalic centers concerned with respiration. Wilson considered the mechanism to be in the region of the mesial thalamus, hypothalamus, and subthalamus. Kelly and co-workers, in turn, postulated that the tegmentum near the periaqueductal grey contains the integrating mechanism for emotional expression. Thus, supranuclear pathways, including those from the limbic system that Papez hypothesised to mediate emotional expressions such as laughter, probably come into synaptic relation in the reticular core of the brain stem. So while purely emotional responses such as laughter are mediated by subcortical structures, especially the hypothalamus, and are stereotyped, the cerebral cortex can modulate or suppress them.”

Interesting facts, animals can laugh:

Sure, life can be funny, and not just for humans beings. Animals laugh too!
Studies by different groups suggest dogs, monkeys and even rats love a good laugh. People, meanwhile, have been laughing since before they could talk.
Acording to scientists neural circuits for laughter exist in very ancient regions of the brain, and ancestral forms of play and laughter existed in other animals eons before we humans came along with our ‘ha-ha-has’ and verbal repartee. Even when chimps play and chase each other, they pant in a manner that is strikingly like human laughter.
Dogs have a similar response.
Rats chirp while they play, again in a way that resembles our giggles. Panksepp found in a previous study that when rats are playfully tickled, they chirp and bond socially with their human tickler. And they seem to like it, seeking to be tickled more. Apparently joyful rats also preferred to hang out with other chirpers.
More study is needed to figure out whether animals are really laughing. The results could explain why humans like to joke around. And there are speculations it might even lead to the development of treatments for laughter’s dark side: depression.
Meanwhile, there’s the question of what’s so darn funny in the animal world, although no one has investigated the possibility of rat humor, if it exists, it is likely to be heavily laced with slapstick. Even if adult rodents have no well-developed cognitive sense of humor, young rats have a marvelous sense of fun.
Science has traditionally deemed animals incapable of joy and woe, although some still regard laughter as a uniquely human trait, the joke’s on them. There is ideed lougter in animal world and it might indicate a close genetic relation of animals that manifest that sort of behavior to human kind since humans, as forementioned exibit similar kind of behavior youngest stages of their life.