Kids laugh an average of 300 times per day. Adults? Only 5. Why is that?
  1. Financial burdens - when you think about paying bills, etc., your mood sours.
  2. Work environments - as much as people claim that work is fun, that doesn't mean it's funny. But many people are in jobs that aren't even fun, and it is reflected in their mood and lack of laughter.
  3. Malaise - days seem to run together into a monochrome memory of busyness. Who wouldn't be bored of that?
  4. Sadness/loss - the older you are, the more likely you have lost someone (or many people) who are important to you. Even though people generally return to a baseline, they are still effected by the endless March of lost friends, family, spouse, parents, etc. and not just to death, but also drifting apart.
  5. Many people are isolated, without a community. Much laughter comes from interaction. Interaction comes from being somewhere where there are other people.
  6. New vs Old - for a child, so much is new, so much is unexpected, so much is magical. You haven't seen this situation before, or heard that joke, or seen someone make such silly faces. Sailor laughs at such a wide range of things, but those same things would barely get a smirk out of most adults.
  7. Distraction - most adults fluctuate between partially distracted and totally distracted, rarely present in the moment. But humor lives in this moment. If you are distracted, you miss it.
  8. Physical wellness - healthy people are often happier, and happier people laugh more. If you are struggling to walk up a flight of stairs, you aren't smiling a lot when you are exerting yourself.
  9. Real time vs anytime - if most of your contact is Twitter or Facebook, you see comments or posts in an anytime format. I rarely smile or laugh when I read comments, or even comics, even if what I'm reading is hilarious. I think it has to do with the lack of transients of the moment; it will be there if I sit for a moment and absorb it.
  10. People aren't funny - when young, you spend time with everyone your age, playing, having fun. If you are surrounded by adults who aren't laughing, your own opportunities to find moments to laugh may seem limited...