The Haunting of Hill House--Repetition

One phrase and idea that is repeated many times through out the book is the "picnic by the brook." We first see the idea in chapter two section two (page 52 in my book) where Eleanor and Theo are getting to know each other. They are inventing ideas to make themselves feel better about the situation and to create a sense of family that each is lacking. Then in chapter six section three Theo and Eleanor are confronted by a supernatural event. They come across a family having a picnic down by the brook. This scene terrifies both girls to the point of fleeing in terror, but why? It is not horrific. It is just a family doing what both girls imagined would be fun. Perhaps, it was not the family that scared them, but the thought that they might never have that moment of happiness and connection with other people. This fear seems to be well founded because in the very last chapter when Eleanor is being sent home she mentions that they have not had their picnic and then she dies. Why is this statement the last thing that she says (other than good-bye)? I think that Jackson is pointing out that Eleanor still desperately wants that "Leave it to Beaver" kind of family, but feels she is losing it. Suicide is then her way of remaining where she feels she belongs. Perhaps, she is the one (or one of the ones) who will now walk Hill House for 80 more years.