why do the sunni and shites hate each other?

Answer 1

Right now they hate each other in Iraq, because since the beginning of the last (20th) century the Sunni's were in charge and didn't give the Shiites any power at all. The Shiites didn't become violent though till the first Iraq war.
The two versions of Islam have been around in the Middle East for centuries. It's not true that for all that time all Sunnis hated all Shiites - and that is probably still not true.
The problems now are (as usual actually) about politics and power and how they interact with the international situation (the US in Iraq) and the tensions between various groups of people.