It is very simple to see things around you that you would like to own, or people you would like to be like. As you get older you might long to be young again. If you were an athlete or runner maybe you long to run once more or to regain the fitness of youth. I know when I am out walking and see a runner in front of my how I would love to be able to raise the pace and catch up or maybe even pass. Such is life. The grass always looks greener.
Reminds me of the story of the jealous crow.