Malbim: Derech is the path for the Rabim. Nesiv is a small individual path. Not only should you not veer your heart to her big path (to be Mezanah with her) - further, do not stray on her individual paths, e.g. to talk with her and look at her. Perhaps he is sure in his heart that he will not sin, and he will receive reward - 'let us pass by the house of Zonos and overcome our desire, and receive reward' (Avodah Zarah 17b).