Sforno on Exodus 19:12/ספורנו על שמות י״ט:י״ב

1כל הנוגע בהר מות ימות, a warning to deter the people from being harmed in the event they would trample the boundary markers around the mountain in their eagerness to catch a glimpse of G’d. Such people, if they were to become victims of their own folly, would mar the entire joyful experience of the giving of the Torah by defiling the earth around them with their carcasses. They would cause their relatives to become mourners on what should have been the most auspicious experience of their lives. The presence of G’d, the Shechinah, does not manifest itself to people in mourning.