Like a sparrow in its flitting, like a swallow in its flying, a curse that is causeless does not alight.
Proverbs 26:2 ESV

Curses have no power that we don't give them, whether through our faith in them or through our lack of in God. In the former case, curses become a self-fulfilling prophecy. We believe them and then begin to behave as if they are already true, sabotaging ourselves and our relationships. In the latter case, our sins remove God's protection and open us to spiritual attack.