When we were younger, we felt that we could literally do almost anything. We can run around without feeling tired, climb trees, do a marathon, stay out all night partying with friends, all on minimal sleep. However, as we grow older, we start to feel tired after strenuous activity, the capacity for our bodies to recover and the mental will to continually do the hard things most will not can start to wain.
Our daily commute to and from work is suddenly tiring and all we want to do during the weekends is sleep in and laze around the house. Unfortunately, this is a normal part of growing older. What used to be a bottomless pool of energy is now a severely limited basin whose reserves come and goes on a whim.