What is this about? Why did Mark include it?
Many believe that the figure is the author himself, inserted as a way of identifying with Jesus or demonstrating that he was an eyewitness to these events. But if we follow the story, we find an alternate explanation for who this young man is.
The next time a "young man" appears in Mark is the end of the gospel, after Jesus' resurrection. Three women go to the tomb to anoint Jesus' dead body with spices. However, when they arrive, they find the large stone rolled away from the front of the tomb and inside the tomb, a young man (not an angel, as in other gospels), dressed now in a white robe. The young man is a messenger with a message: Go and tell.
We can think about the connections between this young man and the earlier young man who fled, leaving his garment behind. The first young man was dressed in an ordinary linen garment; the second young man in a white robe, a symbol of salvation. The first young man left his behind his garment but received a new white robe to wear.
Is this young man Mark's way of narratively describing the process of conversion from old life to new life and the mission that comes from that conversion?
If we consider that the linen garment may have been similar to what would have been worn during a baptismal ceremony in combination with the fact that the young man left it behind at the moment in which Jesus entered into his sufferings and death, and that the next time we see the young man is after the resurrection, where he proclaims the mission of "going and telling" about Jesus, we may draw some conclusions.
Like the young man, we cast off our old life for a new one in Jesus. We put off our old clothes as we put on Christ. The young man signifies the journey disciples go on as we enter into Jesus' sufferings and death in our baptisms in order to be raised into new life so we can "go and tell" others about him.