Wa alaykum salam.
That is mostly depend on the personal idea. The main disagreement between Sunni and Shia was a political struggle. Most of the founders of other thoughts weren't intend to create something new to divide people. They were scholars searching for the truth and had some ideas to solve detail problems. All of them were right at some points and actually they were trying to solve problems.
There is a saying: Being on the side of the king more than the king himself. The people thought that some scholars were more right than the others, so the others should be wrong, thus should be condemned. They may actually be wrong, but that wasn't their intention, they were humans after all. Its like being a fanatic of a football team. They play and you fight.
There are also teachings of some scholars which makes some regular Muslims think false about each other. For example, I heard from a Shia, that Sunnis were on the side of Muawiyah, thus he was very hateful with Sunnis. I don't know about others, but in a Turkish Sunni Teaching, we were told that Ali(ra) was right on that issue. Also in Turkey, there are news on tv occasionally that Iran Shiites are cursing the first 3 caliphs during salat etc. So many people believe such things.
There is an issue in Islamic Societies named takfeer. It means that you are claiming somebody isn't a real Muslim, but a Mushrikh. Although it is very dangerous according to a hadith, there are so many scholars and sheikhs making this act constantly.
I think that the problem is this: The most of the Muslim world is so uneducated, so intolerant that they can't live with the fact that they may be wrong. Or from ordinary person's side, his/her sheikh can't be wrong. So many people see their spiritual leaders as infallible, can't be wrong, all honest.
As for your answer, the people of the kind which see his/her sheikh or scholar is infallible, would answer you that it is obligatory. Because according to their view, ordinary people can't understand Quran, thus shouldn't think with their minds. They should follow the one true leader. Who is this man? Whoever speaks parallel with their prejudged minds.
There is a group of people who claims that Muslims shouldn't be divided and their claim is based on this verse:
Indeed, those who have divided their religion and become sects - you, [O Muhammad], 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. (Surat Al-'An`ām 6/159)
They will tell you that there shouldn't be sects at all.
Also there is a group claiming there may be different opinions/sects about a problem, and you can follow whichever you want according to your need. You can also change your attitude with the change of your need. For example, eating sea products is considered as haram in some sects, and halal in some sects. According to these people, you shouldn't eat them as a safety precaution. But if there is no other thing to eat, you can eat them.
As you see, its all about personal opinions and beliefs.