Holy Spirit reveal to him that he woul not die until he had seen the Lord's Messiah - Luke 2:26
Humans are creatures of anticipation. Serveral studies show expectation of something good benefits our health and sense of well-being
Simon had lived a long life of such close fellowship with God and the Holy Spirit told him he would see the Messiah
A voice told him to go to the Temple, this was his day
When Simon saw baby Jesus, he knew immediately that this was the One he had been waiting for
Simon: Lord, now let your servant die in peace, as you have promised. I have seen your salvation, which you have prepared for all people - Luke 2:29-31