Just as despair can come to one only from other human beings, hope, too, can be given to one only by other human beings.

This quote is saying that Humans can be the source of both despair and hope. Despair might come from someone telling us bad news, or making us feel like our situation is hopeless. Hope can come from someone being supportive, positive and providing encouragement. Ultimately, people can affect our mental state, either positively or negatively.