Because you don't know the horrors of life.
Because you lack the responsibilities of an adult and get to see the world through rose tinted glasses and everything is so much cooler, Why do you think so many 30+ year old's hated the Prequels LOL
Your dreams haven't been crushed yet, you don't have to work or go to school, your parents are your safety net, your biggest worry is which crayon to use, everyone is a potential friend, you can do stupid stuff without people judging you harshly, etc. Ignorance is bliss that's why kids are so happy.
No tension, no depression & best part no work :-D
because time seems to go so much slower than when you are grown.