In "The Boy in the Striped Pajamas," why does Bruno deny knowing Shmuel?

At a moment of weakness, Bruno denies knowing Shmuel and
does not stand up for him.  It is at the moment where Bruno showed compassion and care
by ensuring that his friend had something to eat.  Yet, the moment is interrupted by
Kotler's presence.  In this instant, Bruno recognizes that his transcendent friendship
with Shmuel can be contingent on circumstances.  Showing fear of what Bruno knows is
possible, he denies knowing Shmuel.  It is significant for a couple of reasons. The
first is that it shows how dreadful conditions are at the time.  Bruno is a good friend
and one who does care for Shmuel.  Yet, the awareness of what is happening and the
conditions that surround them reveal themselves in this instant of abandonment. 
Additionally, it is significant because it proves to be an epiphany for Bruno.  He
understands that he should never turn his back on his friend.  While he did so in this
instant, it is a moment that serves as a reminder to never do so again.  This will prove
to be prophetic later on in the narrative, when the stakes are so high and traumatic. 
One singular moment of callousness becomes something never to be repeated again, to both
of their detriment.