I always wondered — it's hinted that Bolt is not just immortal, but that the story universe is just one of a nested series of them that he has created and then entered, so that he's potentially not just immortal but older than the universe (which, if it has progressed far enough to have planets, means billions of years). Presumably there's some limit to his memory capacity. So: maybe the fact that he's always quiet and mostly without personality is because he has long since gone senile, and can't remember most of what he's lived through, and just keeps moving on all the time because he has no clue what else he could do.