I think is symbolizes the passing of a certain period in the narrarator's life (BW) The period in which the songs describe. Or just the passing of youth into uncertain adulthood.
To me, Pet Sounds is THE best break-up album ever! It starts out with the youthful optimism of a new relationship and then goes through all the usual ups and downs and then a break-up and then the guy sees the girl later and her hair is short and she's now hard and mean and bitter and not the same person he fell in love with.
I agree. To me the whole album is about girl problems, and at the end of it - well, life goes on. There's a train outside, barrelling through.