The Talmud teaches that the evil inclination - the insatiable desire within each of us to experiment with the forbidden - is not so foolish as to entice a person to commit a major transgression. It does not tell an honest person to shoplift; that would certainly meet with fierce resistance. Rather, "First the evil inclination tells you, `Do this,' then `Do this,' until it gradually works its way up to the point where you may entirely reject God" (Shabbos 108b).
The usual interpretation is that the first "Do this" is a seduction to commit a minor transgression, and then it gradually works its way up to more serious ones. The armed robber began by stealing a chocolate bar. Rabbi Yosef Schneersohn said that the yetzer hara is even more wily than that. He may begin by recommending "perform this commandment, because it is a perfectly reasonable thing to do," by urging the person to perform commandments because they are logical. "Observe the Sabbath because you need a day of rest after six days of hard work. Give charity because it is only right to help the needy. Keep kosher because kosher foods are healthier." A person thus trains himself to follow the dictates of his reasoning, rather than to do something because it is the will of God. The evil inclination's next step is, "This particular commandment is obsolete. It no longer has any logical validity."
The only way to avoid this trap is to avoid its first piece of advice. We do the right thing because it is right, not because it accords with our personal likes and desires. Therefore, we preface the performance of a commandment with a blessing that states, "I am doing this in order to fulfill the Divine command." While we should try to understand the commandments, to the best of our ability, our understanding of them should not be our main motivation for performing them.