Why does the Giver apologize to Jonas after sharing the memory of war?

In Jonas's first session, the Giver alludes to the pain to come when Jonas questions his belief that memories will be painful:

The man sighed. "I started you with memories of pleasure. My previous failure gave me the wisdom to do that." He took a few deep breaths. "Jonas, he said, "it will be painful. But it need not be painful yet."

Later in his training, Jonas enters the Annex room one day to find the Giver in a great deal of pain; on these days, Jonas is typically sent away without any training. However, on this day, the Giver begs him, "Please, take some of the pain."
He proceeds to transmit the memory of a young boy dying in war:

The boy stared at him. "Water," he begged again. When he spoke, a new spurt of blood drenched the coarse cloth across his chest and sleeve.
One of Jonas's arms was immobilized with pain, and he could see through his own torn sleeve something that looked like ragged flesh and splintery bone.

The memory is a horrid one. It presents the realities of war, of death, and of immense pain. Jonas notes that he would welcome death himself just before he escapes from the memory.
Therefore, the Giver apologizes to Jonas for a couple of reasons. First, he is sorry that Jonas has to bear the pain at all. He is sorry that he, the Giver, has to inflict tremendous pain on Jonas so that the rest of society can be spared from it. And second, the Giver feels guilty for asking Jonas to take this memory on this day so that he can get some relief. His relief is Jonas's torment, and any adult would feel remorse in asking a child to bear that burden.