The loving guard
This novel begins with a Deus ex Machina which is a literary device that simply means that a character's fate is drastically altered, almost as if by the hand of God. For David that seems to be the case, because without this loving guard to feel compassion for David, David's life is pretty much doomed. He is an orphaned Jew with no real life experience who has lived his entire life in concentration camps. The guard represents the sudden change in David's fate.
The journey
It's easy to interpret David's journey to his mother as an allegory. As an allegory, it has striking resemblances to Dante's Divine Comedy, because both involve a journey from hell, through purgatory (like David's wandering through Europe) and an entry into true heaven. For David, this heaven is to be reunited with the mother he thought had died. For David, the reunion with his mother represents true heaven, because that was the thing he wanted most in life but dared not to wish for, because he knew it would probably never happen. Therefore, the journey through Europe is an allegorical "purgatory" where David passes from one season of life to another one.
Saving Maria
When David saved Maria, Maria's family was amazed and thankful, but eventually, it became clear why exactly David was able to perform such a task—he has been through so much suffering already that he is used to having to suffer. This makes him a difficult guest for the family, because he introduces a lot of pain wherever he goes, because the truth of his story is difficult to stomach. Therefore Maria represents David's sacrificial nature, and the whole incident proves that David's suffering has transformed him into a lonely, wandering hero.
The dog
Later in the novel, David befriends a dog during his enslavement on a farm. When the Nazis soldiers almost find him at the German border, the dog distracts the guards so David can pass through. This is a symbolic moment that shows that David has the support of nature. The dog serves as a symbol that the whole universe loves David and wants him to succeed. The dog's name is King, reminding the reader of "King David," the archetypal king of Israel from Jewish folklore.
The recognition between mother and son
When the artist paints David, he notices a portrait of his own mother, and it has magic importance to him even though he doesn't know who it is exactly. He has found the central image of the narrative—a picture of what he wants most in life, which is to be in the warm embrace of his mother. When he arrives at Copenhagen, he sees the woman, and she instantly recognizes him and embraces him (the same idea, but backwards). In other words, he has received what he wanted most in life. This maternal love is his reward for a successful adventure.
