Why that old lady from Titanic smiles so happily looking at the green stone? Her younger self died many years ago in the grinding machine of life.
Why memories are sweet? Was our brain/director making all this years a colorful brilliant biopic of our life so we can enjoy watching it in our head at the dusk of our life? Why nostalgia? What is the reason for it to exist? There seem to be no use of it, just a soothing placebo for old folks, escape from the feeling of sliding down every increasing slope of the end. May be there is no why... May be it's just a side effect of our neural network: we repeat what is pleasant, ergo we remember what is pleasant. The image trained in our Hopfield model became better and better from time just as a consequence of simple physics of it. What surprises me the most is that it's not even truly the happiest moments of life. May be we do not have the same feelings about those because it's impossible to enter the same memory twice, strict uncertainty principles of quantum brain dynamics forbid it, and, boy, do we abuse our memories of those moments... We overuse our memories of the happiest days of the past by frequent remembering, so the peak of happy reminiscing is somewhere in between the most frequently remembered (and initially the most pleasant) moment of your life and the moments you will never remember: the rarely remembered medium-happy moments?