The friendship that is shared between Bruno and Shmuel is one in which their differences are not relevant, as their common finding of their association is all that matters. Their pasts are irrelevant. The only element that matters is that they have found one another. This friendship is one that lives in the eternal present. It is one in which there is complete forgetting of the conditions in which they live, and a complete submersion in their own friendship. I think that this is the reason why Bruno feels that it is "the best thing." It is a statement on how friendship's transcendent qualities can cause people to lose the frustration they experience in their lives and immerse themselves in a shared association with another. For Bruno, it is an opportunity to forget the tension with his sister and his missing his friends in Berlin. For Shmuel, it is a reminder of normalcy in a condition that is devoid of it. I think that this is where Bruno and Shmuel consider their friendship "the best thing." When Bruno's head has to be shaved for lice, he sees it as another way in which he can forge his relationship with Shmuel, reflective of how their friendship is "the best thing" in that they both even resemble one another.
When Bruno returns home from school one day, he discovers that his belongings are being packed in crates. His father has received a promotion and the family must move from their home to a new house far far away, where there is no one to play with and nothing to do. A tall fence running alongside stretches as far as the eye can see and cuts him off from the strange people he can see in the distance.
But Bruno longs to be an explorer and decides that there must be more to this desolate new place than meets the eye. While exploring his new environment, he meets another boy whose life and circumstances are very different to his own, and their meeting results in a friendship that has devastating consequences.