Why are there differences of opinion among the imams concerning fiqhi matters? Is it essential to follow one of the madhhabs?
Question

My respect towards the well known Imams of Islam (may Allah have mercy on them). If the Sunnah way of Rasulallah (sallallaahu alayhi wa sallam) says one thing, why do we have difference of opinions amongst the scholars and why do we have different madhab of the different imams? For example, what was the madhab of imaam Abu Haneefa? He wasn’t Hanafi, I believe. He followed Quran and the Sunnah of Rasulallah (sallallaahu alayhi wa sallam) through the early generation of Islam, the Sahabah, right? So, he was a Sunni Muslim, right? Does that mean he was a salafi? Then, why for example, am I Hanafi? Is it wrong if a person tries to follow only Quran and the practice of Rasulallah (sallallaahu alayhi wa sallam) without following a madhab? Is it compulsory upon a Muslim to be a follower of one of the imaams?.
Praise be to Allah.The Prophet (blessings and peace of Allah be upon him) was sent with the eternal message of Islam, and the Muslims are commanded to follow that which is in the Book of their Lord and that which is in the Sunnah of the Prophet (blessings and peace of Allah be upon him). The Sahaabah (may Allah be pleased with them) used to refer to these two sources, then after the death of the Prophet (blessings and peace of Allah be upon him), they took on the mission of teaching the people their religion. They scattered throughout the regions, and there is no doubt that they were not all the same in their memorising the Sunnah of the Prophet (blessings and peace of Allah be upon him). The fatwas of those who had memorised more were in accordance with the Sunnah, and those that had not memorised as much strove to work out the correct opinion. Hence the differences in the extent to which they had memorised became a cause of the differences between their fatwas. 
Moreover, the same text may have been memorised by two Sahaabis, but each of them understood it differently from the other, and the text itself was such that it could be interpreted differently by each, and each of them strove to understand the text according to what the Lawgiver intended, and one of them was right. Hence the differences in understanding of the text is another cause for their differences. 
Then knowledge spread far and wide, and there emerged in this religion imams of knowledge and guidance, including the four imams (Abu Haneefah, Maalik, al-Shaafa’i and Ahmad, may Allah have mercy on them). So other reasons for their differences emerged, other than those mentioned above, such as their differences as to whether a hadeeth was sound or weak, and their differences with regard to the principles of understanding the text. This is what is called usool al-fiqh. 
The scholars of the Standing Committee for Issuing Fatwas said: 
As for the reasons for the differences among the scholars, there are many, for example: none of them encompassed all of knowledge, so things that were known to others may have been hidden from one of them, and he may have understood the texts in a way that others did not understand them because the clear evidence was hidden from him. End quote. 
Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood. 
Fatawa al-Lajnah al-Daa’imah, 2/178 
Thus the following becomes clear: 
1.It is obligatory for a Muslim to follow that which is in the Holy Quran and that which is proven of the Prophet's Sunnah, and he does not have to follow a specific fiqhi madhhab.
2.There are many reasons for the differences between scholars, and these differences have been compiled in the book Raf’ al-Malaam ‘an al-A’immat al-A’laam, by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), and Asbaab Ikhtilaaf al-‘Ulama’ wa Mawqifuna min dhaalika by Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him).
3.The imams of Islam are not only four; rather they are many, but Allah has caused these four imams to become famous; may Allah have mercy on them.
4.The way of these four imams and other imams of the Muslims is based on following and revering the texts, and they enjoined us to do that and forbade us to imitate them blindly. The one who is pleased with them as imams should be pleased with their way. None of the imams of Islam called on people to adopt his view and give it precedence over the views of anyone else, and Allah has stated that they are above that. Indeed it is proven from all of them that they warned against doing this and advised people to follow the Quran and Sunnah.
5.These madhhabs are like schools of understanding of the Quran and Sunnah. The imams strove to work out the rulings that they thought were closest to the Quran and Sunnah and there is nothing wrong with the Muslim following one of these madhhabs, but that is on the condition that if it becomes clear to him that the Sunnah of the Prophet is something other than what he has learned from his madhhab, then what he is required to do is to ignore the view of the madhhab and follow the Sunnah. This is the advice of these imams, as Imam al-Shaafa’i (may Allah be pleased with him) and others said: If the hadeeth is saheeh, then that is my madhhab.
6.People are not equal in their study of the texts of Revelation and they are not equal in their ability to understand those texts. Hence many Muslims are content to follow these imams. As these four imams became famous and had students who propagated their views, therefore you find some people following the Hanafi or Maaliki or Shaafa’i or Hanbali madhhab. Usually the madhhab of the common folk is the madhhab of their Shaykh in their city or village. There is nothing wrong with what the common folk do, because they are enjoined to ask the people of knowledge. But they do not have the right to denounce others for adopting a different opinion or to issue fatwas or to adhere fanatically to the words of their Shaykh. Rather when the truth becomes clear to him he must act upon it and not do anything other than that.
For information on the views of the scholars ancient and modern concerning this issue, please see the answer to questions number 21420, and 103339 . 
And Allah knows best.