Better to be lonely than dead, which was his other option until she arrived. (Although he would be very difficult to kill if he did not choose to allow it, fighting a war against the entire rest of the world would still ultimately end with him either lonely or dead. And the Endarkened want to kill him for previously established reasons, and everyone else would naturally want to kill him because he is a Dragonbond Endarkened and this is fucking terrifying.)