What causes fights and quarrels among you? Don’t they come from your desires that battle within you? You want something but don’t get it. You kill and covet, but you cannot have what you want. You quarrel and fight. You do not have, because you do not ask God. (bold mine,
James 4:1-2, 1984 NIV)
One thing that makes people stumble is that they desire something, but do not get it, and so they sin to try to get it.
but each one is tempted when, by his own evil desire, he is dragged away and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death. (bold mine,
James 1:14-15, 1984 NIV)
One thing that makes people stumble is their own evil desire. Also,
This is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil. (bold mine, John 3:19, 1984 NIV)
People who have enjoyed their sins, continue to stumble because they "love" the sinful deeds.