Dennis shook his head. "You're not allowed to cross your own timeline, so you'd outlive all your friends and not be able to see them again, and, anyway, even if you did cross your timeline, there would a crowd of yous around all the time, and you'd never get a word in edgeways! I wouldn't like that at all! I think that's why the Doctor never stays places long, so there's always a last chance to come back for a few minutes when he really misses people!"