The assassin of President Garfield was a disgruntled office seeker. Arthur believed the Pendleton Act's merit system would prevent similar retaliations by unqualified spoils system applicants.
The congress was very powerful at the time and needed to be put in line. The two branches were created to control it. The remaining two branches were not specifically enumerated as thoroughly because they were still weak at the time and their power developed over time.
In a general sense, it was because of the oppression that Britain had put over the colonies that led to the Declaration of Independence. Some of the many factors included tax impositions, the Boston Massacre, Coercive Acts, and Common Sense by Thomas Paine.
Moreover, the declaration was written by a committee of five including Thomas Jefferson (primary author), John Adams, Benjamin Franklin, Roger Sherman, and Robert Livingston and ratified on July 4, 1776 in Philadelphia.