How to be as Happy as a 5 Year Old?

Recently, on business, i was enjoying a light lunch on my own across from a stunning Cathedral in Spain. I noticed several children playing with the pigeons, running around etc. And asked myself the question. How can you be as happy as these 5 year old kids? Again? For, it seems, as we age and become more responsible and focus on being able to retire with dignity, we enjoy life less.
What can it be, that takes us away from that unadulterated "joy" in being alive?

Then, on other travels, i note that children and even the adults, in clearly impoverished conditions, also seem to have a joy about themselves. Children playing football with a makeshift ball made from improvised materials like plastic packets and tape. Literally "having a ball".
Then i thought to myself, "they're enjoying themselves more than i am driving past in my Mercedes and going to dinner where i will most likely spend a small fortune on meat and wine, be annoyed that the meat wasn't rare, and the wine was too warm. And not actually have enjoyed the experience!

What is going on here?

The answer is a journey but here is where i am at:

1. We don't live in the NOW anymore. Always thinking of the future or worse in the past.
2. We worry too much. Most of which never comes to pass anyway.
3. We have become too materialistic in the search for happiness. Only become consumers.
4. We get too much news. Mostly how bad the world is.
5. The majority of people on this earth just want to live a happy life.
6. Don't care what anybody thinks of you. They don't!

The list will grow and develop the "how to be as happy", but hopefully my little story will help those who were intrigued enough to click on it.