Why does Mr. Brown visit Okonkwo in Things Fall Apart?

In Chinua Achebe's novel Things Fall Apart, Mr. Brown is the white missionary who has set up in Umuofia and slowly begun to integrate himself into the Igbo society. When Okonkwo returns from his exile, Mr. Brown decides to go and visit him. 

This visit is motivated by the fact the Mr. Brown had recently helped Okonkwo's son Nwoye, who has renamed himself Isaac, leave Umuofia and go study at a teachers' school in Umuru. Given Mr. Brown's close relationship with Isaac, he mistakenly assumes that Okonkwo will be delighted to hear this news about his son. 

Instead, Okonkwo reacts with rage. He drives Mr. Brown out of his compound, and threatens him with severe injury or possibly even death should he ever return.