Why is the Party so determined to make everyone think the same?
The previous answer regarding control is a very strong point. The other idea regarding the Party's need to keep everyone the same helps to enhance the idea of constant war. The party needs constant war in order to consolidate their own power over society. War is an organizing principle that keeps individuals in check and limits their expression. In this line, to keep the primary focus of all of its citizens towards the advancement of the state, a severe limitation on individuality or personal expression is imposed on the citizens. For Orwell, the vision of a government that has managed to wrestle away all hopes of individual articulation in the name of "the state," or "the party," is a reality that needs to be avoided at all costs.
They are trying to make people stop being individuals. This will allow the party to have more control over the whole population.
One of the main things that the Party wants is to have complete control over the whole population. All the Party really cares about is having power. In order to keep power, they have to be sure that no one rebels against them. One of the best ways to ensure this is to make sure that everyone thinks alike.
By stamping out individuality, the Party will be able to make sure that they realize their major ambition -- to keep control in perpetuity.