Sunnis and Shias should not question each other because neither will find answers that are acceptable to both. Consider this as the biggest tragedy of Islam. It is better to follow Islam as was practiced during the time of our Holy Prophet (PBUH) and not get entangled into controversies that followed after him. Ideal would be that both Sunnis and Shias relinquish the sectarian names and only call themselves Muslims. The non muslim world powers will not allow this to happen, though.