though the tone of these two dramas are a little different they both have a similar backdrop. MLs & FLs in both these dramas had a tragic or traumatic event in their teenage years. there are flashbacks scenes etc.
& also the younger part of the FL is played by the same actress in both the dramas.

Both has the same writer, director & even the studio is same.
Writer : Kim Eun Sook
Director : Lee Eung Bok
Studio : Studio Dragon