The reason why believers are not to fix their hope on the things of this world (whether marriage, mourning, rejoicing, buying, or dealings with the world) is that the present form of this world is passing away.
Rather than building their lives on circumstances in this world, believers should build their lives on the glorious promises of God in Christ.
31 et ceux qui usent du monde comme n'en usant pas, car la figure de ce monde passe.