Because God wants us to.
Well he obviously wants it if he is commanding it. But why should I care about what God wants or commands?
The first question you should ask is whether God exists?
If your answer is "no" then whatever I suggest will be ignored. Of course, I, like the author, hold the view that the "no" answer is emotion-driven and doesn't hold up to reason.
If your answer is "yes", the next question is what is the nature of God? Is God a fearsome, jealous entity that will condemn you to eternal tortue if you so much as burp at the wrong time? Well, then the answer is obvious.
Or is God a detached watchmaker who set things in motion eons ago and then went off to do something else? The answer there is less obvious (although one would be wise not to ignore the provided owner's manual.)
Or is God a loving father who wishes only the best for you? In that case, why not obey?
And if this is the case, for those who choose not to, I'd imagine he'd grant their wish -- which ultimately would mean being without Him for eternity.