1 Peter 2:11

Beloved, I beg you as sojourners and pilgrims, abstain from fleshly lusts which war against the soul. 

The devil will tempt us into bad behavior and help us justify it in our minds.  The war is for our soul and our future here on earth.  The lust of the flesh is like a detour to a dead end.  We end up wasting time, energy, and do damage to ourselves and others when all we want is the best in the world; we can only find that in God and His word.