What is the reason for Ali to become a radical, fanatic muslim?
On some levels, Ali experiences a disenchantment with his Western life. Notice in the opening lines of the story how Parvez comes to understand that something "is wrong" with his son. The possessions he used to value are all disposed of, the room that used to be the symbol of his privacy and personality is transformed, and the British girlfriend and his friends are no longer present. Each one of these reflects that there is a disengagement with "the West." Materialism that is so...
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Obviously, Ali is disappointed of his father's life, because he cannot see any sense or worth in living it. For Ali, Parvez only seems to live for "Western" pleasure (alcohol, prostitutes...), he does not see any philosophy of life behind Parvez's lifestyle. Ali simply does not understand his father's relationship to Bettina, for example.
Ali simply needs a perspective in life - and he has found one in religion. Praying and feeling solidarity to a big Muslim community comforts him and the strict rules give him a structure for life he can follow.