why was reuven upset danny was learning german?

It is not so much the fact that Danny is learning German that bothers Reuven. What disturbs Reuven is how much Danny teaching himself German reminds him of the story Reuven's father told about Solomon Maimon. Danny hs a brilliant mind like Solomon. Solomon also became interested in learning more than just the Talmud and taught himself German because German was "great scientific and cultural language." After learning German, Solomon spent less time studying the Talmud and abandoned his wife and child. Solomon was never at peace after learning German. Reuven is afraid that Danny's study of Freud will lead him down a similar path.