Simeon (and Anna) had waited so long for God's Promised One. Simeon had been promised that he wouldn't die without seeing the Messiah. When Mary and Joseph brought Jesus to the temple, the Spirit affirmed that Jesus was God's Chosen One. Simeon believed and rejoiced because God keeps all of His promises, from least (the personal one) to the greatest (for all people). We have a reason to rejoice because Jesus shows that God keeps all of His good promises for His people.