Why Can't People Just Respect Others Beliefs?

Today in class we were talking about 9/11 and how people are all fighting over the differences in all of our beliefs and that we can't respect each others beliefs and how we have to take everything to the extreme and it was a very opinionated class. So we got really into it. It just really bothered me that people in the United States are bombing abortion clinics, just a lot about class today bothered me. But anyways, some of the stuff we learned was that Muslims were separated into two major groups, either the Sunni's or the Shiites. About 75% - 90% o Muslims are Sunni and 10% - 20% are Shiites. Also, you can either spell their holy book as Koran (English spelling) or Qurán (actual spelling) We also learned that in Buddhism they don't have a god or multiple gods, they just have nirvana which is them reaching enlightenment and getting rid of suffering and a bunch of other stuff. They don't really have a holy book either they just follow the Four Noble Truths and the Eight fold Path.