why is it so bad for a Christian to love worldly things?

Loving worldly things is Another way that Satan seeks to ensnare us is by inducing us to love this world—unrighteous human society alienated from God. Jesus called the Devil “the ruler of the world” and said: “He has no hold on me.” (John 14:30) May Satan never get a hold on us! We realize, of course, that “the whole world is lying in the power of [that] wicked one.” (1 John 5:19) So the Devil could offer Jesus “all the kingdoms of the world” for one apostate act of worship—something unequivocally rejected by God’s Son. (Matthew 4:8-10) The world ruled by Satan hates Christ’s followers. (John 15:18-21) No wonder the apostle John warned us not to love the world!

John wrote: “Do not be loving either the world or the things in the world. If anyone loves the world, the love of the Father is not in him; because everything in the world—the desire of the flesh and the desire of the eyes and the showy display of one’s means of life—does not originate with the Father, but originates with the world.

1) It is evil to love the gift more than the gift-giver.
2) It is evil to love the gift to the point of destruction for you or others.

it can hinder their relationship with God.