conflicts have existed for 1500 years and are not just something that can be overlooked. The fundamental differences date back to approximately 630-650 CE, when the prophet Muhammad died and a successor was elected by the people of Medina. They elected Abu Bakr (father of Muhammads favorite wife)Not every one accepted him as their leader, those who didn t, followed Ali ibn Abi Talib(Muhammadís Son in Law) whom Muhammad named heir.
Another major difference is the practice of taking a temporary wife or mut ah allowed by the Shia but long since eradicated by the Sunni...also they have a difference in spiritual leaders, holy days and cities, as well as the practice of taqiya(the act of concealing ones faith for protection and self preservation. although at the core they share the same beliefs. Tension was heightened further but Saddams rule he and other Sunnis oppressed the Shiites for years...As for the Kurds I still can t find out why everyone seems to hate them...They follow another sect of Islam...Sufi I believe.
Basically there disagreement is much like the fight between different sects of Christianity...for the most part they agree on everything, but are unwilling to accept the little differences that set them apart. Below are two websites that talk about their religious and cultural differences...I hope that this helps you to further understand the longterm animosity between the groups.