Those MUSLIMS who consider Shias as Non Muslim, they are just simply Deaf, Dumb and Blind.

If shias are not Muslims, they would have not allowed in Makkah or Madinah in first place. They would have not allowed to worship in ANY MOSQUE in ANY MUSLIM COUNTRY.
This proves that Sunni Scholars in Saudia, Egypt and other Muslim Countries belives that Shias are Muslim same as Sunnis.

The fact is that there is no difference between Shia and Sunnis in basic beliefs or in the worship they offer. There are certainly differences of detail...

BUT such differences exist between Sunni Schools of Islamic Law and no one suggest they are different in any way. There are four orthodox sects or schools of jurisprudence among the Sunni Muslims .... Hanafiyah (followers of Imam Abu Hanifah), Shafiyah (followers of Imam Ash-Shafii), Malakiyah (followers of Imam Malik) and Hanbaliyah (followers of Imam Ahmed Bin Hanbal).
Also, Wahhabis (Muwahhidun), Ahl alHadith, Ahl alSunnah, Ahmadiyyah, Qadians etc....

So one should understand that if we point OUR finger on someone we should not forget that our own 3 fingers are pointing back to us. Thus raising a huge question?

In short, in my opinion, the reason for the spread of sects must be that people have abandoned the Quran in favour of the opinions of their leaders they idolise.

I feel sympathize for those who never strive for Unity, Love and Peace.