What is so bad about admiring women? Shouldn't I appreciate my wife?
The source of the restriction of not looking at a woman if she is immodestly dressed is the Biblical verse, "Do not stray after your eyes." (Bamidbar 15:39)
The Torah's purpose is not to deny us the joy of a meaningful relationship with a woman. In fact, marriage is considered the ideal state of man. In creating Chava as the ezer k'negdo, the Divinely-appointed helper of Adam, Hashem confirms that "It is not good for man to be alone." This relationship is meant to help every individual live at the highest spiritual level he or she can reach. From this shared goal, children will be brought into the world who will continue as Ovdei Hashem, servants of G-d.
Rather, the purpose of the Torah's laws of Shmirat Einayim is to help us give our full love and attention to our wives. The awareness that your wife is the only woman permitted to you strengthens the unique and special bond of love between you.
Once we understand the benefits of the laws, we realize that they are not restrictions, but safeguards to enhance the quality of our lives. We cherish these Divine laws because they are the cornerstone of a successful marriage and the foundation for a healthy family.
Men, in general, have a strong natural desire for women. In addition to preserving our special relationship with our spouses, the Torah gives us guidelines so that these inclinations do not rule our lives. They save us from distraction, and even from destruction. If "destruction" seems like an exaggerated term in this context, consider the words of the Rambam.
Fully aware of the natural tendency of men to minimize the relevance of these laws, he writes:
There are some transgressions for which a person is not motivated to repent because he considers them trivial and of no real consequence. Among them is the transgression of looking at women. A man who looks at a woman [feels he has done nothing wrong and] inwardly protests, "Did I have relations with her? Did I get close to her?" He doesn't realize that gazing at women who are forbidden to him is a transgression because it leads him to inappropriate conduct... (Hilchot Teshuvah, Chapter 4, Law 4)