The sefirot of "action" engender love and fear on our part, the two "wings" which we need to soar up to the heavens.
As God reveals Himself to us through the sefirot of chesed (kindness) and gevurah (strength), we react with the emotions of love and fear. Just as we have shown kindness and strength to be the two fundamental "actions" of God, so too are love and fear are the fundamental "reactions" to God's deeds.
Since the sefirot of chesed and gevurah are the first sefirot of action, i.e. the visibly perceived activities, the also evoke a reaction on our part. Love of God (ahavah) and fear/awe of God (yirah) are two emotions which the Torah lists as basic to the worship of God, and they are two important commandments in their own right.
To get a very good idea of the interaction between love and fear, and how uncannily they are mirror images of chesed and gevurah, let us turn to Maimonides' description of these two emotions. Maimonides was a towering giant in the field of Jewish law and philosophy, although it was not known whether or not he involved himself in the study of Kabbalah. In his work "Mishneh Torah," he describes the obligation of loving and fearing God in the following manner:
The Lord Who is esteemed and awesome: one is commanded to love Him and fear Him, as it says, "You shall love God, your Lord," and "You shall fear God, your Lord."
What is the appropriate manner to love Him and fear Him? When a person contemplates God's most wondrous creations and deeds, and sees therein His Wisdom which is unending and incomparable, the person immediately begins to love and praise [God] and he is overwhelmed by a tremendous desire to know the Great God. As King David said, "My soul thirsts for God, the living God." And as a person thinks about this point, he is immediately thrown back and filled with awe, realizing that he is a small, tiny benighted creature, standing before the Perfect Intellect. As King David stated, "When I behold Your heavens, Your handiwork, [I ask], Who is man that you remember him?" (Rambam Yesodei HaTorah 2-1,2)
We see in Maimonides that love and fear are reactions that exactly parallel to chesed and gevurah. Love is the desire to expand, to broaden one's self. Fear, on the other hand, is mode of contraction, of imploding one's personality into the realization of God's overwhelming greatness.
The Zohar states that love and fear are two "wings" without which Torah does not "soar up" to heavens. This means that there are two emotional components in the worship of God. When a person does an act that is uninspired, insipid, it falls flat. When a person does an act with feeling and understanding, it comes alive. Just as physically a person who is motivated appears alive and animated, so too does a mitzvah done with love and fear comes alive.
WORKING IN TANDEM
But how can love and fear/awe act in concert, if they are opposites? The fact that the Zohar likens them to "wings" means that they act in tandem. While a person may hop on one foot, it is difficult to envision a bird flying with only one wing.
Man must find himself in the mitzvah, and he must find God in the mitzvah.
The answer is that every mitzvah is a bond between man and God. As such, the attitude towards the mitzvah must relate to both these points. Man must find himself in the mitzvah, and he must find God in the mitzvah. Love is the mode of the person finding himself in the mitzvah. Awe is the mode of finding God in the mitzvah.
When I want to marry a person I love, it is because I have a sense of being fulfilled by this person. When I express love of mitzvot, I am demonstrating the sense of my personal fulfillment in doing the mitzvot. I have found an element in the mitzvah that speaks to me; that adds to my person. My inner drive for self-enhancement grabs eagerly onto the mitzvah and I seek to enrich myself with its content.
Let us now look at fear/awe. A young man is dating a young woman and is taken by her intelligence and personality, and he starts to feel love for her. But during the second meeting, she is more expressive and he is overwhelmed and awed by her brilliance. This awe is an appreciation of her qualities rather than his wants and needs. This awe relates to her qualities that are beyond the qualities that he discovered in her the first time, and they reveal to him a higher level of quality. For that is awe –- the awareness of something much greater than myself.
When awe/fear kicks in, the person realizes the Divine nature of the mitzvah in which he is engaged, and is overwhelmed by it. This "overwhelming" does not dissuade the person from doing the mitzvah but, rather, gives him a further appreciation of its content. Thus, love expresses the person's appreciation of the mitzvah, fear gives him a still higher awareness of the mitzvah, and then love once again yearns on for this appreciation, etc.
There is another mitzvah wherein love and fear combine in a most similar fashion, and that is in the mitzvah of honoring one's parents. We are enjoined to "honor your father and mother," and also "your mother and father shall you fear." The Talmud explains that "honoring" one's parents includes such activities as standing up for them, helping them eat and get dressed, etc. -- all positive activities.
"Fearing" one's parents, on the other hand, includes not sitting in their designated seats, nor referring to them by their first name, and not blatantly contradicting them. All of these are negative activities, acts of restraint.
On the one hand, the good we have received from our parents –- including our very existence –- means that we are acknowledging that the source of benevolence and our existence comes from them. But this very self-same awareness forces us to be awed by their presence. Inasmuch as we owe our very existence and so much of our "selves" to them, our sense of self is so much diminished, for what we have is not really ours.
Thus, love and fear are the mirror reflections of chesed and gevurah. God's chesed gives us what we have, and we love Him for it. On the other hand, God's gevurah demands accountability and truth, which makes us realize that what we possess is not really ours, and we become most keenly aware of God's Omnipresence and our insignificance.