I think that ET first became aware of us when we started testing nuclear weapons. However, there may have been other earlier missions to this planet as human technology matured. Once we demonstrated that we now had the means to destroy ourselves, we became more of a threat to the entire human race. Even if life is common in the Universe, I think that sentient tool users like humans are probably relatively rare. So, a benevolent ET might intervene to keep the human race from destroying itself to give us a chance to mature past our warlike nature and magical beliefs.