Once again, they werechosen by God. Another example could be that Reuven was chosen to catch Danny's ball, Danny was chosen to pop Reuven in the eye, and their following friendship was chosen as well. God is a tricky being. It seems that he likes playing games with his people. Look at Job if you want an example.
Lets look at what is perceived by the title of the novel.
For a great while we believe that Danny is chosen to continue his father's footsteps as rabbi. Surely, this is true, because this is the way it has been for generations. But perhaps, just perhaps, Danny was chosen to break the mold. Remember, every once in a while God likes to throw a curve ball, just like Reuven. Danny knew that someday he would have to tell his father of his plans, and when God threw Reuven out there, Danny connected, and followed through.