It is well-known that Sunnis treat the closest companions of the Prophet (i.e., Abu Bakr, Omar Ibn Alkhattab and Othman Ibn Affan (Radiya Allahu Anhum رضي الله عنهم اجمعين)) with the greatest of respect. Why then do the Shi'a tend to dislike them and treat them with such contempt?
I know some Shi'a do not (e.g., the Zaidi's in Yemen, the Shi'as in Southern Iraq and the Arab Shi'a in Ahwaz,) but many do dislike them. They won't even name their sons after Omar, Abu Bakr and Othman; it is almost impossible to find Shi'as with one of these names, whereas they are very common names among Sunnis.
I have also heard that Abu Lo'lo'ah (أبو لؤلؤة), the killer of Omar Ibn Alkhattab (RA), is actually venerated as a hero in some Shi'a schools, whereas the Sunni consider him a criminal.
Please note: I am not looking for an in-depth analysis here, just a summary of how this substantial difference in opinion came to be.