Time to leave.
Jesus had been on the earth for forty days after his resurrection from the dead and had been seen by over five hundred people and his disciples. But it was now time for him to return to the father. He had to go, because only by his going could the promised Holy Spirit be poured out on the new believers and the new dispensation of the Way.
Mark 16:19-20, Luke 24:50-53, Act 1:9-11.