John Gill

And he said unto them, why are ye troubled,.... Who had more reason to rejoice, and be glad, as they were when they knew that it was Jesus: and why do thoughts arise in your hearts? whether what they saw was Jesus, or an apparition, which gave them a great deal of trouble and uneasiness, and filled them with fright and terror; as it was, and is usual with persons when they fancy they see a spirit, or an apparition; see Mat 14:26.