Why was Bruno proud of his father after hearing Maria's story in The Boy in the Striped Pajamas?

Bruno is proud of his father after Maria's story because
in his mind, it shows that his father is a good man.  In many ways, Bruno feels an
allegiance with Maria as he sees her as an outsider, like he is.  Thus, when she speaks
of his father's generosity and benevolence to Maria's family, he is filled with pride. 
At a point where Bruno has stood against his father's decision to move the family to
Auschwitz as well as the father's value system, Maria's story reminds Bruno that his
father was a good man.  This is something that fills him with pride, as it means that
while Bruno may disagree with his decisions right now, there is a core goodness within
him. This is where Bruno is once he hears Maria's story.  It allows him to feel pride
and love towards his father, even if right now he does not understand the full
implications and rationale for what he is doing at Auschwitz.  In this light, the story
also brings complexity to the father's character.  There is a goodness within the
family, and just as Bruno cannot fully reconcile this benevolence with what is happening
with the family right now, we, as readers, have to reflect on how we are viewing a man
who is a high ranking Nazi official.  We must reconcile our view of him as a murderer
with the reality that he does demonstrate goodness.  As Bruno is confused, we are too. 
This makes his feeling of pride more poignant, as in the world of confusion there is the
feeling of pride that a son feels towards his father.