Not all Ponies love Hearth's Warming, some of them don't wish to celebrate and they have their reasons for their disliking of the holiday.

This thing haunts Twilight's father, and it soon becomes an infliction on the entire family when Night Light recognizes a pony he once knew and cared for like a brother.

LESSONS:

1: "Some Friendships-- once broken-- cannot be amended."

2: Sometimes an apology or wanting to make things right just won't cut it.