In the ultimate evil figure, fantasy shows us something completely alien to what we know/experience. You are trying to put human qualities and thought patterns onto a being that is not human. The Dark One is not the same as humans, and so although it may get lonely and boring for humans, the DO probably views it in a completely different way. You can't understand a being whose nature is destruction. Their goals are not meant to be understood, because ultimate evil cannot be understood. Especially when it is at a higher level (in the DO's case, the level of a deity) than humans