oomis:
Prior to Yaakov Avinu, there was no sickness before one died. Rather, the soul just left the body as it had been breathed in, through the nostrils, with a sneeze, so to speak. Yaakov acknowledged the “gift” of being sick before death, so that one can put his affairs in order. A sneeze is no longer a sign of death, but just a reminder of illness (sometimes a symptom and sometimes just a reminder). This is why people say ASUTA or LIVRIYUT, or something like that, to a person who sneezes. But the sneezer himself quotes the words of Yaakov Avinu. And even though people are more familiar with ASUTA (or whatever someone else says when someone sneezes), it is closer to a requirement for the sneezer himself to say LISHU’ATCHA KIVITI HASHEM. [from Rabbi Yehuda Leib Ginsberg, shlita]