Well it is all about nostalgia, isn't it? I was 15 when the album came out and still full of dreams of what the future might hold. The summer of 1967 has almost mystical associations for me and you can tell me that Sgt. Pepper isn't the best album ever until you turn blue in the face and you'll never convince me.
It's not the music, it's the yearning for times past which will never come again.