You are happy when you are young. Because you think that the future will be much more bright and much more fun. You think that you will have a lot of money in the future and you will enjoy your life. This makes you happy.
As you grow older, you begin to realize that you cant change things and your life is the same as before. You can see in the future and you see that nothing is going to change. You realize that its not worth thinking about the good future. You suddenly realize the reality.
The essence of youth is generally lost when the idea ‘anything-is-possible’ is replaced by ‘a-lot-less-is-possible’ then I originally thought and probably isn’t worth bothering with.