Lady Macbeth does not try to sweet talk him into sticking to the plan. Instead, she berates him and questions his manhood. She does not spare his feelings, and calls him a coward. Lady Macbeth is well aware of her husband's weaknesses, so she knows which buttons to push to spur him into action. A light touch would not have had the desired effect, so she attacked his masculinity to bolster his courage.
Macbeth needed her at this point, because his greed and desire for power were not strong enough by itself to go through with the murder.