The central conflict in Prisoner of Azkaban is Choices. At the start we see how mcuh Ron loves his rat. He gets some rat tonic for it because it is sick and he cares for it lots. Enough that when he even gets a slight clue that Scabbers is dead he turns completly on Hermoine leaving Harry stuck in the middle of them trying to get them talking to each other.
Harry has been shown in the previous books how much he misses his family and in this one we see just how much. He isn't afraid of Sirius even though he's heard the rumors about the 12 people Sirius killed. Once Harry finds out the truth he ignores al those closest to him and goes for vengeance on Sirius which is never completed due to Harry finding out the truth. Harry then is left with the choice of whether to let Pettigrew die as revenge for his father or do the right thing. In the end the choice remains Harrys as it always does.
The cultural value is identified in the choices we make everyday. Every little thing we do influences something that will happen in the future. in PoA this shown very well with the time-turner, Buckbeak, and Pettigrew.