Because you become jaded. There is a lot less wonder as you get older, things become more stable and you begin to fill your time with responsibilities. Time spent doing work is not "real time", so it seems like you have a lot less time. As we get older we spend more of our time relaxing/doing nothing, and we seem to not have enough time to do what we want/need to do. Become more active, have new experiences and time will slow down a bit...