Could be related to skin orgasms. Some people have brought it in relation with foreplay, cuddling and delousing. Perhaps the closeness of the sounds evoke strong associations with being actually touched resulting in something like a laughter response to being tickled (which could be thought of as a signaling that there is no danger from a spider crawling down your neck). Perhaps the high spectra in whispering make it even related to chills we receive from high-pitched noises, which is possibly related to teeth maintenance. I'm not aware of any research.