Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the Islamic College in London and also the Managing Editor of the Journal of Shi'a Islamic Studies.
Most Muslims who follow a specific sect or ideology do so because they genuinely believe it is the correct one. Since we do not have the Prophet (S) here present with us today to tell us which beliefs and practices are the most authentic, everyone has to do their best to try to discover that. So it is less about making sects and more about following what one thinks is most true.
(Of course, what we consider to be true or authentic is strongly influenced by the ideas we live around or grow up around, the ones that are shared by people around us, etc. However, the sincerity is usually there.)
For that reason, it isn't possible to impose one view of what Islam "is" on all Muslims and expect them all to agree on it. There are some areas today where we just have to agree to disagree.
Of course, in some cases, sectarianism may also be due to other factors such as politics, national identity, racial factors, or personal agendas, as well as the influence of Shaytan, and this sort of thing would be condemned.