In the edited book, Worldliness, C.J. Mahaney explains in the opening chapter why worldliness is so serious. After showing the importance of loving Jesus above all else biblically, and after debunking the idea that fighting worldliness is legalism, he ends the first chapter of the book with a powerful, brief sentence summarizing it all. He writes:
"Worldliness is so serious because Christ is so glorious."
He explains this further by writing,
"We must fight worldliness because it dulls our affections for Christ and distracts our attention from Christ."
Although succinct, I believe this insight is gold. Christ is amazing, beautiful, wonderful, and worth living for–he is glorious. He is who we were made to behold and display, for his praise (because he is great!) and for our joy (as we get to revel in how great he is!). But worldliness–loving and clinging to things here apart from him–dulls our taste for him and distracts us from him. This is a tragedy.
Therefore, worldliness is serious. It seriously hurts us and those around us as we are trying to find satisfaction at things we weren't made to and points away sadly away from Christ. And this is so sad because he truly is so incredibly glorious.
Quotes from C.J. Mahaney, Worldliness: Resisting the Seduction of a Fallen World (Wheaton, IL: Crossway, 2008), 35.