R. Yonah: If one wants to go on a straight path, 'one who comes to purify himself, [Shamayim] helps him' (Yuma 38b).

R. Yonah: A person can carry out his intent in practice only if Hashem prepares his steps. How can it cross his mind to steal, extort and sin against his G-d?! This is like a king who passed by a lame man. He had mercy on him, and carried him in his arms in the path that he wanted to go. The man stuck his hands out and was cutting the king's nice trees and shooting arrows at the king's beloved, and all about whom the king commanded 'anyone who harms him is like one who harms my eye' (Zecharyah 2:12). Is there brazenness, folly and treachery greater than this?!

Malbim: When the root 'Chashav' is a verb form, it refers to unusual or improper thoughts. "Lo l'Adam Darko Lo l'Ish Holech v'Hachin Es Tza'ado" (Yirmeyah 10:23). Man chooses which path to go on, but Hashem moves the limbs - the final action. This includes the tongue, which expresses thoughts in speech - "l'Adam Ma'archei Lev ume'Hashem Ma'anei Lashon" (16:1) and the legs, which go on the path that he chose. Therefore, "Lo l'Adam Darko"; if Hashem does not desire his path, He can stop him. "Me'Hashem Mitz'adei Gever Konanu v'Darko Yechpatz; Ki Yipol Lo Yutal