God has always been infinite and perfect in His aesity. This includes aspects such as glory, love and fellowship – as the eternal Trinity, God always already had these things maximally!
Neither can the infinite be reduced by creatures refusing to give God His due – as CS Lewis put it, the sun doesn’t shine less bright just because a madman denies its light.
So what can be gained by creating us? I postulate it’s a QUALITATIVE matter. God created free creatures with agency in order yo enjoy something that by definition He could not have without doing that – namely, the glory and and love and fellowship of beings who are NOT GOD.
We cannot add or take away quantitatively to God’s glory… but we can give Him a qualitatively different glory.
THE SCOTTTCAST 
July 10, 22 at 11:47 am
I don’t remember subscribing to your blog, but since you’ve been kind enough to share your views with me via email, I’d just like to point out that the biblical narrative, and theism in general, are rationally unjustified and are therefore not reasonable to accept as true. Philosophically speaking, there are two hidden assumptions in theology that are never justified: 1) that everything which exists materially, to include quantum particles and dynamics, are finite, and 2) that an abstract, immaterial object can exist wholly independently of the cosmos. Since neither of these assumptions are justified, it isn’t rational to believe theism is actually true. Have a nice day!