based on Swami Radhanandaji's discourse on Gita

In the previous shloka, Arjuna asked Lord Krishna a pertinent question that arises in our mind all the time: "Why does one commit sin, even if one does not wish to? Why are we helplessly carried away towards sin? What is the root cause behind all the sins?"

Lord Krishna explains here that the root cause behind all sin is desire, lust and passion (kamna). Unfulfilled desires cause anger and force us to commit sin. The best definition of sin is: The actions or thoughts, which culminate from desires, that do not take or lead us towards God. The end result of all our desires is sorrow, misery and disquietude. If the desires are partly fulfilled, then we desire more and grow greedy. Our biggest enemy is desire, and its appetite is vast. But desires channeled in the proper direction can become our best friend.

One should strive to find how deep rooted and widespread his or her desires are. If not controlled or reined properly, they can consume us. Desires attack only when we are not vigilant and initially make us unconscious by overpowering our intellect. It spreads like cancer but has a farther reaching effect than cancer because it destroys not only our present body or life but many more to come. Lord Krishna also explained  the root cause of desire and how it consumes or destroys a person in Ch. 2:62-63.

We don't have to give up hope. Gita and Lord Krishna are our guides. God has given us discriminatory power and wisdom to control our intellect, mind and organs and thus to control our desires.

Time and again man gets humiliated because of his being victim to lust and greed. Still he is unable to recoil from them and give his thought to God. -- Sri Ramakrishna