If God is self-sufficient, why does He need us?

And second: What is the purpose of creating intelligent beings and making them to chose between living according to God’s rules or not?
What I really want to know is God’s motivation for creating us.
The analogy with the parent and a children is the closest thing to the probable answer (in my opinion). But then I ask you: What is the purpose of creating children that will never surpass you? (by definition the human condition is far from perfection). This would be (in my opinion) a paren’t greatest joy that his children are better then him.

Related posts:

1. I want to be self-sufficient from my parents, need a high-paying job with HS diploma only?
2. Is there a limit in honoring ones parents?
3. I Want Self Sufficient Living!