Repeatedly in the PoD Cadsuane remarks that she "has to make that boy smile" in reference to Rand. Why does she have to make him smile? Does she want to make herself feel better, does sh have an unrealised talent for comedy? NO!!! It is not in the nature of Cadsuane to do something for the heck of it. She has a purpose in mind when she remarks (numerously) that she must find a way to make Rand laugh. In a slow progression Rand has gone from a simple farm boy to a king. Along with this transition he has simultaneously lost any sense of humour (except when he laughs in madness). The more monarch like Rand has become, the greater the effects of the taint. Perhaps that is just a coincidence with the fact that he uses the power more, but I think there is something additional going on. One must also remember that Cadsuane seems to know more about the taint than anyone except perhaps the Forsaken. It was Cadsuane who warned Rand that Callandor multiplies the effects of the taint when used as a Sa-angreal, and it was Cadsuane that asked Rand about the voices. Hence, it will be Cadsuane who will teach Rand and the Ashaman that laughter is the best medicine to slow the effects of the taint.