Bismillah al-Rahman al-Rahim
In the verses that describe the creation of human beings, God says he has made humans in a beautiful form. Because of this, they have an obligation to behave in a beautiful way in their lives and in their relationships. By doing so they will be acting in accordance with the form and nature with which God has created them. The Qur'an says: “Do what is beautiful (good), as God has done what is beautiful (good) to you.” (Qur'an 28:77)
When someone behaves in an ihsan manner, in a beautiful (good) manner - the benefit is not to God, who is above all harm or benefit. Rather they do ihsan to themselves, to their own souls because such behaviour conforms with the true form in which they have been created - there is a harmonizing between what they create with their actions and the deep nature with which they have been created. “If you do what is beautiful, you do what is beautiful to your own souls, and if you do what is ugly (asa), it is to them likewise.” (Qur'an 17:7)
So husna (beauty) is the reward for those who behave with ihsan. The Qur'an says, “As for him who has faith and does wholesome works, his recompense shall be the most beautiful.” (Qur'an 18:88) Here also we see that beautiful behaviour is recompensed with something even better, with an increase in beauty. “Those who do what is beautiful will receive the most beautiful and increase.” (Qur'an 10:26)
But this is not to say that because our creation and our form and our nature was with beauty, that this beauty appears automatically in our behaviour and character. It requires awareness, consciousness, and a following of God's guidance. It requires us to awaken potentials that are within us but which may be dormant. We make the effort towards this through increasing our consciousness about everything we come into contact with. God provides the fulfillment as a blessing, a baraka. This is the intention of the dua which says: “O God, You have made my creation (khalq) beautiful, so also make my character (khuluq) beautiful too.” (hadith)
Unselfish parental love is a shadow, in some ways, of God's care of us. In many Qur'anic verses the first beautiful (good) act that is required of human beings after refraining from shirk (associating anything as equal to God) and observing Tauhid (God's essential unity), is to do what is beautiful and good to their own parents. It is through parents that we receive our creation, that we come into existence. Parents are the means that God employs in creating people. By the demand of Tauhid, God is the actual, foundational source of creation, but parents are the means employed for that creation. Parents are the touchstone, the portal, through which we enter the world and they are the protectors and helpers who can provide without thought of return to their children. We are as dependant on them in our early years as the embryo that clings to the side of the womb which gives it life.
And just as God made human beings as a whole emerge from the womb of the universe so also he makes individual human beings emerge onto the stage of this world through the medium of parents. They have a noble function, a high role, which is also a very fundamental, basic role that is in accord with the very nature with which Allah has created humans. The parents (as a pair) have participated in and fulfilled a profound act that symbolizes with what takes place in Allah's creation as a whole.
When the Qur'an describes something descending from another world, another level of existence to this world, it uses the word tanzil (descent). In this sense, revelation descends, and in fact the type (or archetype) of everything that exists has descended from “Allah's storehouses” or treasuries to this world. The Qur'an also uses the word tanzil to descibe the rain descending down from the skies. The mingling of the rain with the soil is referred to as a marriage (nikah). This is used figuratively to describe the coming together of various aspects of creation. From this intimate mingling something new springs forth - the earth brings forth flowers, herbage - it opens to new creations, new life, new potentials and also nurtures them to fruition. The marriage of the rain and the soil is the first step in this act of creation, this act of of parenting. The act of marriage, and of parenting, according to the Qur'an, courses through all things.
Now, it says in the Qur'an that all of Adam's children from the first to the last, were brought in front of Allah and were asked “Am I not your Lord?”. This implies a pre-existence at some level for every human being who has ever been born or ever will be born. The point of entry for these human souls to come into the world, is their parents. The parents are the conduit, the means through which a human soul descends from that other plane of existence to this world. So the parents are the means by which tanzil (descent) occurs - by which the human soul descends and enters this world and begins its material existence.
There is a powerful use of symbols and phrases in the Qur'an which, taken together, make it more clear why exactly the Qur'an places beautiful behaviour towards parents right after the necessity for Tauhid. The very concept of parenting as an act which resonates with other acts that occur throughout the creation begins to become apparent - the metaphysical underpinning to a biological role is partially revealed. And so, as it does repeatedly in verse after verse, the Qur'an overlays physical, psychical, and spiritual aspects like a layered blueprint that provides a comprehensive picture of its subject. So we need not dwell only in the basement of an exclusively material view of existence, but can expand our horizons to encompass the multi-layered depth and vastness of reality. 
“And worship God and do not associate anything as equal with Him, and behave in the most beautiful manner with parents....” (Qur'an 4:36)
1 – This article arose from a curiosity (followed by an investigation and contemplation of possible reasons) as to why the Qur'an lists respect for parents immediately after emphatically declaring God's Unity (tauhid).