No, cooperation is not always voluntary but sometimes it may be enforced. According to the conflict perspective of Karl Marx - cooperation is enforced and not voluntary.

The viewpoint of Durkheim and Marx differ on cooperation yet they both emphasize cooperation.

Marx argues that society is divided into various classes and so the cooperation cannot be voluntary in the society.

The cooperation is the result of division of labour; it is the alien force which enforces cooperation.

As per the rule of division of labour, a work is divided among various persons on the basis of their specialisation and the main goal can be achieved only when all the works are put together. So, to achieve the result people are compelled to co-operate.