One reason Jesus stayed on earth for 40 days after His resurrection instead of ascending immediately into heaven was to demonstrate to His followers that He truly was alive.
After all, they knew the Roman authorities had put Jesus to death, and that His body had been taken down from the cross and sealed in a tomb. And when that happened, they were filled with despair and fear; many even went into hiding. They had believed Jesus was the promised Messiah - and now their hopes were shattered. They had forgotten His promise that He would return from the grave, and they felt they had no future.
But when Jesus appeared among them after the resurrection, their lives were changed. The greatest miracle in all history had just taken place: Jesus Christ was alive! During those 40 days, He appeared to various groups of disciples, proving beyond doubt to them that he had been raised from the dead by the power of God. Over two decades later, the Apostle Paul wrote that "he appeared to more than five hundred of the brothers at the same time, most of whom are still living" (1 Corinthians 15:6).
Another reason, however, why Jesus stayed on earth then was to teach His disciples, and prepare them for the task of telling the world about Christ. Is your faith in the risen Christ, and are you seeking to share His message of salvation with others?