Some of The Earliest Witnesess Didn't Believe So Why Should We?

In Matthew 28:16-17 we read: "Then the eleven disciples went to Galilee, to the mountain where Jesus had told them to go. When they saw him, they worshiped him; but some doubted." How is it possible for some of the eleven disciples to doubt after seeing the resurrected Jesus? Then too the Apostle Paul claimed in I Corinthians 15:6 that, "After that, he [Jesus] appeared to more than five hundred of the brothers and sisters at the same time..." Of them, only 120 believed according to Acts 1:15: "In those days Peter stood up among the believers (a group numbering about a hundred and twenty)." Even by the testimony of the New Testament itself many of the earliest witnesses didn't believe. Why should we?