Why does smiling, laughing cause my severe pain from c.r.p.s. to lessen substantially?

The gate control: theory by Melzack and Wall . When the nociceptive (pain) signal arrives at the gate, it is subject to modulation by descending neuronal signals from cortical areas that act to amplify or reduce the intensity of the nociceptive signal coming from the periphery. In your case smiling and laughing are pain inhibitors at the gate. Hope it makes sense.