Why does one have to become like children to enter the kingdom of God

What is it that children have that we as adults have lost as we grow up? Children are pure at heart, meek, humble, ready to forgive and forget, they dont care about ruling over others (in their eyes all children are equal), not selfish(?) and most important of all they are dependent 100% on their parents to provide for them. It is their faith in their parents that is most commendable. In the same way i think Christ expects us to depend on him totally for everything. Right from salvation to how we live our lives as his children we are to depend on him totally.
David as a kid had enough faith to go kill Goliath. Can we say we have enough faith in God as David had when he was a kid. Will you be able to go kill Goliath now, if you had to? For some reason, as we grow up, our faith decreases. We start looking to our increased abilities to help us out rather than to God who is our sole provider.
All of us can learn a thing or two from children. They are the closest thing to angels we get on this side of heaven.
By the way, I dont think Christ meant we have to become childish, but only child like.