Especially, but not only in the US, it's a bad custom to scare people with all sorts of scary masks and even find it funny. What few people think about is what such rather childish pranks can trigger in those affected: spontaneous defecation (which is very humiliating for those affected), cardiac arrhythmias in people with predispositions, heart attacks, and even anger that someone had the nerve to do such a thing to you. Don't get me wrong: I LIKE good, successful pranks and find them just as amusing as most other people! However, successful pranks don't come at the expense of other people's dignity...and so some of the reactions of those who are pranked are entirely understandable.