Answer:
It happens to the best of us, and sometimes we don't even know why it happens, but it does. Some say it can be medicine for the soul and for others it may happen at the wrong place and at the wrong time.
Laughter for the most part, seems to be a mystery, and I wouldn't blame anyone for not knowing the answer, because to tell the truth there's not just one reason for laughter.
In general there are many different reasons why we laugh (e.g. you may have been told a funny joke, harboring good emotions, or simply have been tickled a little) No matter the form, laughter's still involved.
Laughter is caused by outside stimuli that translates to the brain and it then begins to create hormones. These hormones whether it be dopamine or some other chemical such as oxytocin, may cause physical reactions and movement in the body. Depending on what is making a person laugh, the hormone will vary. When being tickled, is different, because it is an involuntary, fight or flight reflex. (This is why you can't tickle yourself and expect to feel it.)
Ignore whatever comments I sent earlier, because I just found out how to edit responses. Don't judge I'm just slow.