Firstly, let’s understand first the cause of emergence of differences:
the Prophet (blessings and peace be upon him) was sent with the eternal message of Islam, and Muslims are commanded to follow that which is in the Qur’an and the Sunnah of the Prophet (peace and blessings be upon him).
the Sahabah (the Prophet’s companions, may Allah be pleased with them) used to refer to these two sources. After the death of the Prophet (peace and blessings 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 memorizing the Sunnah of the Prophet (peace and blessings be upon him). The fatwas of those who had memorized more were the most consistent with the Sunnah, and those who had not memorized as much, strove to work out the correct opinion.
hence, the differences in the extent to which they had memorized became a cause of the differences between their fatwas.
Moreover, the same text may have been memorized by two Sahabis, 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 are another cause for their differences.
then, knowledge spread far and wide, and there emerged 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 hadith was sound or weak, and their differences with regard to the principles of understanding the text. This is what is called Usul al-fiqh.
there are other many reasons for the differences between scholars, and these differences have been compiled in the book Raf’ al-Malam ‘an al-A’immat al-A’lam, by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him).
a Muslim should observe the most opinion supported by sound evidence even if this opinion is different from his school of fiqh. This way the wide gap between Muslims would narrow and the wished unity becomes attainable.
anyhow, we should consider the above causes of difference of scholars’ opinions so that we avoid the bad effects of imitating or adopting a definite school without the other and not to raise fanaticism toward special school which is a wide way of troubles among Muslims.
today, the Muslim world is going through severe ordeals that are really enough to make Muslims unified and joined in face of their enemy, which are already unified in face of them.