The problem is that children here are too molly coddled, parents are afraid to let them out to play for fear of abduction, children don't want to go out because of all their technology, health and safety has gone crazy so children can't even play proper games on the playground (we are a fearful nation, we live in a suing society), throw into the mix that if such and such has an iphone then they need an iphone, the bullying that is rife in schools and on social media, a lot of children seem to have a poor me attitude - these children i've found usually have no parents at home when they get home from school, so these children just need attention. When a child doesn't have all the lastest stuff they become outcasts, throw into the mix the junk food that they're eating with all the additives, preservatives, sugar, fat and salt content and parents who in some families both of them have to work, and school/nursery starting so early for our children compared to the rest of the world, it's no wonder they are unhappy, in other countries like Africa that Ammi has mentioned, the children are happier because they are all the same, they are as poor as each other, they have experienced the same as each other so are thankful and grateful for their lives, there is no health and safety preventing them from doing things, if they want to climb a tree or play conkers (probably don't have conkers!) but they can, they don't have adults preventing them from exploring their surroundings, they have freedom, they don't have social media so there's no bullying or bragging, the meals they eat are wholesome, fresh and adequate (i know that Africa is a very poor country and a lot of children are malnourished, i'm talking about the children in the areas that are happier, as in the OP), you can bet that these children have no access to McDonalds or Burger King, and in some countries children have a part to play in their families, maybe helping with cooking, farming, looking after siblings etc yet these children are happier, i think happiness comes from being valued and being valid, adults need to understand that children need guidance, support, love, freedom, boundaries, structure, and to feel that they matter, i think we need to get back to basics, but it'll never happen.
http://news.bbc.co.uk/1/hi/education/7234578.stm