what is the misunderstanding between keller and paul?how the misuderstanding influence them?

I think their biggest misunderstanding comes from the fact that Keller refuses (or can't) express his experiences during the war verbally, that he doesn't share his past with Paul without reservation. On Keller's part, this might denote an inability to communicate, but on the other hand, it might just be that this information is so personal he doesn't want to share it. I've always believed that Paul was too pushy in his attempts to learn about Keller's past..... but that is my own personal opinion. What do you think?