The yetzer hara has a trick to get us to sin. Sometimes he makes us believe that without us giving in to these desires, our lives feel dark, void and empty.
A person could have the most fulfilling life. A wonderful wife, beautiful children, a good job, a good spiritual environment, goals and ambitions. His life is full and he is feeling happy. Suddenly, for some reason, lewd thoughts enter his head, or a beautiful women walks by. The yetzer hara can take way all his energy, ambitions and fulfillment for a few seconds, or even sometimes a few minutes. He suddenly feels a deep void, a neediness, and a darkness enters his soul. Suddenly he feels that all he has in life is worthless. He just wants to let go and stray after his eyes, pleasure himself, and fall back into his old habits ands addictions.
How can this be? Does it make sense?! We must realize, all it really is, is a trick of nature to get people to reproduce. It is animal instinct. It is a chemical reaction. And that is why it is so important to guard the eyes. Otherwise you are letting the yetzer hara play his tricks on you again and again, until you will ultimately fall for it.
But even if we saw what we saw. We must remember that it is a trick. We are stronger than that. We won't give up true happiness and long-term fulfillment for this silly chemical reaction. When your mind goes into this state of neediness and darkness, tell yourself that it's a lie that you are unhappy and needy. You really ARE a happy and fulfilled person. It is only chemicals that are telling you suddenly that you aren't happy and that you need these things. Be strong for those few minutes. Don't give in, and you will feel SO much happier later on.