The reason love for the world excludes love for God is that all that is in the world is not of God. In other words it's just empty talk to say that you love God if you love what is not of God. John could have rested his case at the end of verse 16. Don't love the world because love for the world can't coexist with love for God. But he doesn't rest his case here.
Nobody buys stock in a company that is sure to go bankrupt.
Nobody sets up house in a sinking ship. To set your heart on it is only asking for heartache and misery in the end. If you share the desires of the world you will pass away. You will not only lose your treasure. You will lose your life.