In the book "Lennon remembers" by Jann Wenner, "The dream is over" represents Lennon's stance that the myth "the Beatles were God" had come to an end. "If there is a God," Lennon explained, "we're all it." According to the wikkipedia.
I like the line. I have a bit of a differnt interpretation; maybe he was trying to make a point to Paul McCartney.
I suppose that might have been a difference between John and Paul. Paul kept of pushing out pop fodder; there was never to much thought behind what he wrote in his lyrics. The dream was still on for him.
"We had a dream when we were young men, we actually lived it; and guess what? We're no better off than when we began. "The dream is over". There is no going back Paul.
I like the line because it scales our significance. Even John Lennon, one of the most influential people of a generation knew this. Even with his, not my interpretation. Great people and insignificant come to a realisation often 'the dream is over'.
Most people come to a point where 'The dream is over'; I came to that place 2-3 years ago; a pastor; I could have kept going, networking, doing the latest and greatest; but in my heart I knew. Others come to the point through a divorce, a deep friendship that goes cold; even ugly; lots of reasons....