After the Doctor gained his new regeneration cycle, he was in a "reset mode" of sorts, reverting back to his younger looking Eleventh Doctor form for a short while. During this time, he imparted to Clara that people are always changing every day, and while that is a good thing, people need to also remember who they were as well. Lesson Learned: Change in people's lives can be good or bad and the past helps to define who you are as a person. Remembering what happened in the past can always help who you are in the present.