It is involuntary. When the brain takes in information and processes it as something that causes pleasure or strikes an amusing chord, the immediate response is a laugh. It is the physical response to funny information through the senses. For example, when a stand-up comedian delivers a line that is generally perceived to be humorous by all or most listening to his or her act, that line delivered, then processed by the audiences brain will produce laughter, but, if a joke is not relatable to the masses, there will be little or no laughter. We all have "funny files" in our brains. Things that have amused us over time will continue to do so, time and time again, thus producing laughter, the physical response. Of course, it is possible for people to fake a laugh, but in doing so, they are not feeling the inner pleasure that an involuntary laugh brings. Then, there is the unfortunate truth that some people simply have no sense of humor and couldn't or wouldn't laugh at anything, no matter how funny it is. Funniness is also genetic. It runs in families and even a person's ethnicity can influence their "funny factor". Example, the Irish and Jewish are known for their senses of humor, Maybe not so much among the Japanese or Belgians, for example. And, when was the last time a Dutch guy really cracked you up? By the way, this is my own opinion, not meant to be racially biased, just a perception through my many years of living and being around funny or not so funny people.
