10. The Fourth Doctor
"Well, of course I'm being childish! There's no point being grown-up if you can't be childish sometimes." - Robot
The very next story after the Third Doctor's moving line about fear comes perhaps one of the most oft used Doctor Who quotes. Tom Baker's debut story as the Doctor, 'Robot', features this marvellous line in which the newly regenerated Doctor lets viewers know exactly what sort of a man he is. Perhaps one of the most joyful lines in the show's history, this exclamation sets the tone for Baker's tenure and gives a glimpse into the old/young dynamic that many actors would later try to play up to in their own interpretations of the classic role. Baker imbues the line with such playful glee that one can't help but feel excited at the possibility of indulging one's inner child just as the Doctor seems to whenever he's given the opportunity. Perhaps the ultimate comeback to Sarah Jane's protestations of "You're being childish!" - indeed, the newly regenerated Doctor displays a great deal of playfulness which was previously unseen in the programme and allowed the writers to inject a great deal of humour into the stories from this point onwards. Perhaps one of the most innocently motivational utterances in the show's history, this all time classic line is undeniably a fan favourite which only serves to give us Whovians license to act like big old kids!