How do they choose to be sunni or shia?

Answer 1

The main difference between Shia and Sunni Muslims is the case of Caliphate and Leadership after Prophet Muhammed:

Shia Muslims believe that Imamate/Caliphate/Leadership after prophet Muhammed was appointed by him (peace be upon him) and the holy prophet appointed Imam Ali ibn Abi Talib is the leader and successor followed by 11 Imams who are known and appointed by the holy prophet Muhammed and the previous Imam.

Sunnis, however, believe that Caliphate is done in different manners:

- Case of First Caliph:

By election (through a small committee (Saqifah) in the case of Abu Bakr.

- Case of Second Caliph:

Appointment by previous caliph (Abu Bakr) in case of Omar.

- Case of Third Caliph:

An election between nominated number Muslims who were less than 10 (nominated by Omar) in the case of Othman.

- Case of Fourth Caliph:

Majority of Muslims elected and chosen, in case of Ali ibn Abi Talib (1st Shia Imam).

- Case of Fifth Caliph:

Appointment by previous caliph, in case of Hassan ibn Ali (2nd Shia Imam)

- Case of the Sixth and the rest...

Then it became kingship even if the previous king is overthrown by power in case of Ummayads and Abbasids, etc.

Answer 2

by beliefs about successor of prophet.

Shia and Sunni have mostly same beliefs and both believe in fundamental beliefs of Islam. Today there is up to 260 sects in Islam that are in two main categories of shia and sunni.

the conflict of shia and sunni has been always amplified by world Imperialism to prevent Islam from gaining power.

Shia believes in Allah, prophet and all fundamentals of Islam.

Shia pray 5 times a day but usually do the 2, 3 and 4, 5 pray together and so may seem they do only 3 pray a day. there is some differences in details of pray like genuflect (for shia should be on a stone or part of earth and for sunni on carpet or floor ).

shia believe after death of prophet God selected the successive for prophet who is the political leader of Muslim community and people can not and does not the right to selected it.

Shia believes the Caliph (Representative of God in earth) is selected only by God and can not be selected by people because God said in Koran: "Indeed, I will make upon the earth a successive authority" (Quran 2:30) or "O David! We did indeed make thee a vicegerent on earth" (Quran 38:26) also other verses.

Shia believes such verses means only God can select an Islamic political leader (Caliph).

In Shia Islam Caliphs after prophet are 12 Imams and all of them are the same and have no mistake and have the knowledge of everything (not absolute knowledge like knowledge of God) and they hear all sayings and even thinks of all humans after even after their death by permission of God and they are intermediates between God and human Shia Muslims always support them and forgive their lives for them.

Shia Muslims do not consider selection of Abubakr as Caliph by people valid because God did not select him. They believe God ordered prophet to declare people that Ali is selected as successor of prophet and prophet did this mission many times during his prophet-hood and mainly in Ghadir event after last Hajj of his life at a 3-4 hour speech in front of 120,000 Muslims at Ghadir in hot desert after 2 days stop of long caravan for gathering and 3 days after speech for congratulations and homage of Muslims to Ali. Islam at that time had high power in world and Some companions had high interest in leadership position after prophet. Muslims knew that it is the final Hajj of prophet (predicted by prophet) and were going along with prophet to hear what prophet says at end of his life. Shia believes Ghadir event is the most important event of Islam and is mentioned in Koran in many verses like "Today" (اليوم) in verse 5:3 of Quran is the day of Ghadir. Or Verse 5:67 (O Messenger! proclaim the (message) which hath been sent to thee from thy Lord. If thou didst not, thou wouldst not have fulfilled and proclaimed His mission. And Allah will defend thee from men (who mean mischief). For Allah guideth not those who reject Faith) is revealed at Ghadir day and is about declaring political leadership of Ali after prophet.

Both Sunni and Shia Muslims accept happening of Ghadir event But different interpret. Some Sunni writers tried to reject the event of Ghadir as declare of leadership of Ali and said at Ghadir prophet only wanted to say Ali is my friend and no one should bother him. Ali had 3 different missions to Yemen by prophet during 10 years and in 2 of them some companions of prophet had some conflicts with Ali and and prophet said some sayings about Ali to solve conflicts and some sunni writers have mixed stories of missions of Ali to Yemen with story of Ghadir to prove Ghadir event is not about Leadership of Ali and is not important.

Shia scholars believe some Sunni writers have changed and deviated historical evidences about political leadership of Ali in old historical books and republished them to destroy evidences of leadership of Ali from old Sunni books.

The famous Shia book Al-Ghadir (الغدير) by Allameh Amini is a collection of evidences and proofs for Ghadir Events written after referring to 100,000 Sunni books and full reading of 10,000 sunny books. Allameh Amini is a famous Shia scholar and spent 40 years of his life in travelling to access original old Sunni books in libraries in different countries to write this 20 volume book only from Sunni books and not using any Shia book. Some Sunni scholars tried to reply this book but then said if we want to reply this book we should first destroy all books of ourselves.

Shia doctrine have root in Karbala tragedy. When tragedy of Karbala happened most of Iranians understood there is two different kinds of Islam and after happening of Karbala tragedy most of Iranians became followers of "Ahl al-Bayt" and forgive their lives for them like what they did in Iranian Revolution and Iran-Iraq War. Shia Muslims have at least 120,000 Hadith (saying) from The Fourteen Infallibles that is the base of Shia Islam along with Quran. Shia believes according to " Hadith of the two weighty things" The Fourteen Infallibles are the only valid interpreters of Quran and sayings of them are from God because they are Representative of God in earth and they do not say anything from themselves. Shia believes 12th of them (Mahdi) is alive and because people have killed all 11 Imams before him, he is in hide like Jesus and will come out of hide toghether with Jesus when people become ready and want to accept his leadership. All 11 Imams have been killed and during their life thy all have been in prison or under hard control of government to not have any political action. They never had enough serious followers to can get political power. Among them only the sixth Imam Ja'far al-Sadiq was relatively free in his life to have only scientific activities (but banned from any political action) and so only he is known in west because he established university and had students from all over the world. Westerns know him as a polymath: an astronomer, alchemist, Imam, Islamic scholar, Islamic theologian, writer, philosopher, physician, physicist and scientist. He was also the teacher of the famous chemist, Jābir ibn Hayyān (Geber), and of Abū Ḥanīfa, the founder of the Hanafi school of Sunni Islamic jurisprudence. He never wanted be a famous man and only shared his knowledge with who wanted to learn. Some researchers consider his teachings the root cause of renaissance.

Sunni Muslims also accept Mahdi and his worldwide leadership as Caliph of God in earth at end of world. There is a famous Hadith from prophet Muhammad (PBUH) that Mahdi at hide is like sun behind cloud, he is hidden from people but people receive his benefits. Shia Muslims say some faithful believers have connection with Mahdi and use his knowledge. Shia Muslims believe that it is proved at least 1000 persons have had connection and talk with Mahdi during his hide period but Any one having relation with Mahdi keep it and does not declare it in public until is alive.