Not the most scientific source, but hey, she cited HER sources. This little article on Hillbilly Housewife explains why a good belly laugh can be better for you even than smiling at something that gives you pleasure.
Evidently it works better if your brain has to work at it. It's also supposed to work better when there are not deep, sad connections to your own life, but I have had really good results from laughing -- deeply, unkindly, and with unholy gusto -- at my own misfortunes and idiocies. So don't hesitate to crack wise (among the right people, of course) about your own sorrows, if you can and are so inclined.
I am not seeking sympathy, and don't want to appall those of you with tender sweetness in your brains, so I will withhold my most recent hyena-giggles. However... I have never gotten such a good laugh from my mom as several years ago, when I shocked myself unconscious by putting a wet sponge between one hand and an electrical outlet and my other hand in the stainless steel sink and called mom to ask her if I'd die when I woke up. The electricity blew an exit wound in my hand and stopped my heart for a moment, and I woke up feeling kind of lousy and hella stupid. Thing is, I had decided that I should not touch the outlet to wash it with my hand, because it was, you know, electric; instead, I decided to put the sponge between me and it because it was rubber and thus non-conductive. So I dipped it in the tap to get it WET, and...
Mom hung up on me because she was laughing so hard she dropped the cell phone in her lap (she was driving, this was years ago before we all knew how dangerous it is). Then I called back again, got my grandmother in the passenger seat, with my mom shrieking laughter in the background. Grandma laughed so hard she hung up, too. On the third call they managed to tell me to call the E.R. and ask a nurse if I was in danger (my heart was hammering).
The E.R. nurse, with perfect Chicago south-side aplomb, listened to me with sympathy. She told me that if I felt WORSE I should come in, but probably if I was doing all right I would be okay. As I was going to hang up the phone, she told me, "Now, Linda, sweetie -- listen to me. Are you listening, honey? Don't you EVER do that again."
And you know, years later I find it hilarious. Heck, it was pretty funny then and there. Whether it has healed more pain than it caused, well, who knows? But Mom and Grandma got a really good laugh together at it.