Why are Muslims divided into sects or different schools of thought?
If you want to get the proper answer about the above issue you have to go through the following all three points:
1. All Muslims in the world should be united
It is a true that today Muslims are divided amongst themselves. The real truth is that such divisions are not endorsed by Islam at all. Islam believes in protecting unity amongst its followers.
The holy Quran/Koran says:
"And hold firmly to the rope of Allah/(God) all together and do not become divided. And remember the favor of Allah/(God) upon you - when you were enemies and He brought your hearts together and you became, by His favor, brothers. And you were on the edge of a pit of the Fire, and He saved you from it. Thus does Allah/(God) make clear to you His verses that you may be guided."
Which is the rope of Allah (only one god) that is being referred to in above verse of the Glorious holy Quran/Koran. The Glorious holy Quran/Koran is the rope of Allah/(God) (only one god) which all Muslims should hold fast together. There is a huge emphasis in this verse. Besides saying 'do not become divided' as well as it also says, 'hold firmly to the rope of Allah all together'.
In the Glorious holy Quran/KoranAllah/(God) further says,
"O you who have believed, obey Allah and obey the Messenger and those in authority among you."
All the Muslims should follow the The Glorious holy Quran/Koran and authentic Hadith(Statements or actions of prophet Muhammad) and be not divided among themselves. Be careful about false Hadith.
2. It is also fully forbidden to make sects and divisions in Islam
In the Glorious holy Quran/KoranAllah/(God) says:
"Indeed, those who have divided their religion and become sects [like Shia, Sunni, Sufi etc.] –you, are not [associated] with them in anything. Their affair is only [left] to Allah; then He will inform them about what they used to do."
In this verse Allah/(God) says that we should disassociate our-self from those who divide their religion and break it up into sects.
But when we asks a Muslim person, "who are you?", the common answer is either 'I am a Shia', or 'I am a Sunni'. Some answer are Maliki, Humbali, Hanafi, or Shafi. Many of them say 'I am a Deobandi' or 'I am a Barelvi'.
3. We should respect all the great scholars of Islam
In past communication system was not as improved as today. It was very hard to gather all information together. At that time there was no search engine and no internet. Even paper was very limited. On the other hand any one can do any unintentional error. In modern time it is very easy to verify any information which was very hard in past. We must respect Islam's all great scholars, including the four great Imams, Imam Shafi, Imam Humbal, Imam Abu Hanifa and Imam Malik. They were great scholars and may Allah/(God) reward them for their hard-work and research. One can have no objection if someone agrees with the views and research of Imam Abu Hanifa or Imam Shafi, etc. But when posed a question, 'who are you?', the reply should only be 'I am a Muslim'.
If you want to know more about Islam you can watch following videos: