Was Islam always sectarian?

When did Islam started to be viewed/accepted in a sectarian way. Most of the Islamic sects are very sectarian in nature. They believe that only they possess the correct interpretation of Islam and are guided and that only they will be among the people who will enjoy eternal happiness and bliss in paradise?

I think the way Islam has evolved into various sects is very natural because it's just human nature to have opinions about something. Probably the absolute worse thing that has happened in the Muslim world is the narrow-mindedness and the lack of tolerance displayed towards followers of other sects. When or how did the intolerance started? Or perhaps intolerance was a big part of the Arab culture and it eventually made into the official version of Islam under the guise of 'This is what the Prophet said and he's always right'!?

I've been observing this student of the Sunni Imam Abu Hanifa by the nickname of Abu Yusuf for a while now. I think he played an important role in illustrating Islam via the many Sunni sects. He legitimizes the process of how various opinions (even somewhat contradictory) can be consolidated into Islamic theology. I don't fully agree with this approach of labeling everything 'theologically sound'.

I'm still waiting to write something more substantial later but I'll leave this note here

As a disciple of Abu Hanifa, Abu Yusuf's doctrine largely presupposes that of his mentor. His writings and prominent political positions helped advance the Hanafi school of Islamic law throughout the Islamic empire.[3] While most of his legal opinions (fatwas) were firmly rooted in the doctrine and methodology espoused by his former teacher, there are some points on which he diverged and revealed his own legal thought. The doctrine of Abu Yusuf was more dependent on traditions (ahadith) than his master, in part because there were more authoritative prophetic traditions available to him in his time.[1] He also reacted against the somewhat unrestrained reasoning exhibited by Abu Hanifa. However, he was not always consistent; in a certain number of cases he disregarded sounder and more highly developed doctrine by diverging from the opinions of his former teacher.[1] Based on his surviving works and opinions, certain tendencies in Abu Yusuf's reasoning have been determined, such as his tendency to logically follow the implications of a proposition to an absurd conclusion (Reductio ad absurdum) and his use of rather caustic language in his attacks on opponents' positions and in defense of his own.[1] Abu Yusuf is also noted for the frequency in which he changed positions on various issues, which has been suggested is a result of his experience as a judge.[1] Abu Yusuf's greatest legacy is in affirming and advancing the Hanafi legal school as the predominant source of legal thought in the Islamic empire and providing a legal framework for defining and restricting caliphal authority in regard to fiscal policy.

Reference
[1] http://en.wikipedia.org/wiki/Abu_Yusuf