One that time there was a very huge cultural and linguistic variation between the citizens of East and West Pakistan. Citizens of East Pakistan were culturally more close with Indians(Indian Peoples) especially with citizens of West Bengal.
They had their own language totally and their language was completely different from our national language Urdu and they considered it as their single identity. The citizens of East Pakistan were very educated. They had done a lot of work in literature like poetry, style novels and small stories etc. was done in Bengali language. So they took their language as their national identity and symbol. They were not ready to be taught or even speak Urdu and to accept it as national language. They claimed that because of their majority according to the population Bengali should only be the national language of both East and West Pakistan and that were the first difference rose between citizens of both sides.