Why are sunnis and shitte Muslims divided?

Sunnis and Shia Muslims are divided primarily due to a historical disagreement over the rightful successor to the Prophet Muhammad after his death in 632 CE. Sunnis believe that the community should select its leaders, leading to the election of Abu Bakr as the first caliph. In contrast, Shia Muslims hold that leadership should remain within the Prophet's family, specifically favoring Ali, the Prophet's cousin and son-in-law. This initial schism has evolved into significant theological, political, and cultural differences over the centuries.