"In Retrospect" is about being true to yourself. It reminds you to stop thinking that you need to be all of these other people or that you need all of these things that you don't have, but instead to ackowledge the great person you already are, and all of the wonderful things that you do have in your life. By Dr. Fillasho having Dennis and Rita envision themselves as the different people in these grand scenarios, the two of them eventually realize this ideal. By accepting who they are, they redevelop a love for themselves which, in turn, helps the love in their relationship.
This is a message that can resonate with everyone on a different level. It's so easy to be your own worst critic, but instead of knocking yourself down for everything that you're not, why not build yourself up for everything that you are?
As one of our 101 year old friends said the other day "Look forward to tomorrow, but enjoy today." Be you, and be proud of it.