I have a theory. At least, for my self. I think I start losing them when I begun getting hard spankings at home. I think my laughs began to vanish when I coursed through school with stern teachers or bullying classmates. Then, by the time I was out of school, I had jobs or businesses to think about, difficult people I had to deal with, appointments to keep, and wife and children to take care of.
I love watching children laughing. It reduces stress when I'm able to laugh along with them. I think children laugh so much because they are like "fresh bread from God." Innocent. Pure. The laughter of children reminds me of what needs to be redeemed from inside all of us.
In the darkest of times, I discovered that laughter can be a savior. It produces something within that revolutionizes our perspective. Like sunshine, laughter can drive away typhoon from our faces. G.K. Chesterton once wrote, "Angels can fly because they take themselves lightly."
Try it. Nurture it. Get healing from laughter.