Why do people say, "bli ayin hara," or "kenina hora"?

An "ayin hara" means an evil or begrudging eye. It is believed that an envious or begrudging glance is able to cause evil to the person at whom it is directed. According to a statement in the Talmud, 99 out of 100 die of an evil eye. Hence, we use the expression in Hebrew "bli ayin hara," or in Yiddish "kenina hora" - meaning, without a begrudging eye, when a person's health, wealth, intelligence, success, etc., are being admired.