Why is it so easy to be happy when you are a child?

I remember I was so excited when i was 10 years old that I was getting a new bed! It wasn't even a cool toy or anything, but just a bed lol. Now, I wouldn't really care if I were to get a new bed, but I remember I was so excited when I was younger about so many things my stomach would hurt with anticipation. I never get that feeling anymore... Also, roller coasters were so much fun, but not anymore... Why does this feeling go away as we age?