This quote comes from the beginning of Chapter Four as the two mysterious voices continue to discuss Ender and what they need to do to him to hopefully mould him into the soldier they want him to be. This quote's importance lies in the way that it emphasises how all of Ender's existence is spent as something of a guinea pig or a laboratory mouse. Everything about his life is one elaborate experiment as Graff and those above him eagerly watch and follow how he reacts to certain situations. He is not treated as a human boy with his own complex bundle of emotional needs and concerns, but instead is treated dispassionately, like something out of a Chemistry class, as Graff seeks to find the combination that will cause him to develop and mature into the independent, creative thinker who will be able to lead the armed force of the humans against the buggers and be able to destroy them.
When we remember this ultimate purpose, this does help us to understand Graff's carefully considered abuse. He above all does not have the luxury of considering the emotional needs of Ender if the fate of humanity is hanging in the balance. Desperate times call for desperate measures, even if the casualty is one young, innocent boy.
Ender is being carefully manipulated to become the perfect commander who will save the human race. For this to happen, Ender needs to outsmart and out think the buggers. Ender's advantage is his creative thinking. The mysterious voices in the beginning of the chapters have decided that isolating Ender from the rest will ensure that he doesn't adopt their way of thinking. But they have to be careful of his isolation to not drive him insane.