(Just a short list – we could add so many, many more)

“Give me Liberty, or give me Death!”
Patrick Henry, March 23, 1775

We know through painful experience that freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed.
Martin Luther King, Jr., Letter from Birmingham Jail, April 16, 1963

• from the statement by eight Alabama clergymen that prompted Dr. King’s response:
We are now confronted by a series of demonstrations by some of our Negro citizens, directed and led in part by outsiders.  We recognize the natural impatience of people who feel that their hopes are slow in being realized.  But we are not convinced that these demonstrations are unwise and untimely).

• From Dr. King’s response:
We have waited for more than 340 years for our constitutional and God given rights. The nations of Asia and Africa are moving with jetlike speed toward gaining political independence, but we still creep at horse and buggy pace toward gaining a cup of coffee at a lunch counter. Perhaps it is easy for those who have never felt the stinging darts of segregation to say, “Wait.” But when you have seen vicious mobs lynch your mothers and fathers at will and drown your sisters and brothers at whim; when you have seen hate filled policemen curse, kick and even kill your black brothers and sisters; when you see the vast majority of your twenty million Negro brothers smothering in an airtight cage of poverty in the midst of an affluent society; when you suddenly find your tongue twisted and your speech stammering as you seek to explain to your six year old daughter why she can’t go to the public amusement park that has just been advertised on television, and see tears welling up in her eyes when she is told that Funtown is closed to colored children, and see ominous clouds of inferiority beginning to form in her little mental sky, and see her beginning to distort her personality by developing an unconscious bitterness toward white people; when you have to concoct an answer for a five year old son who is asking: “Daddy, why do white people treat colored people so mean?”; when you take a cross county drive and find it necessary to sleep night after night in the uncomfortable corners of your automobile because no motel will accept you; when you are humiliated day in and day out by nagging signs reading “white” and “colored”; when your first name becomes “nigger,” your middle name becomes “boy” (however old you are) and your last name becomes “John,” and your wife and mother are never given the respected title “Mrs.”; when you are harried by day and haunted by night by the fact that you are a Negro, living constantly at tiptoe stance, never quite knowing what to expect next, and are plagued with inner fears and outer resentments; when you are forever fighting a degenerating sense of “nobodiness”–then you will understand why we find it difficult to wait. There comes a time when the cup of endurance runs over, and men are no longer willing to be plunged into the abyss of despair. I hope, sirs, you can understand our legitimate and unavoidable impatience.
Martin Luther King, Jr., April 16, 1963 – from a jail cell in Birmingham, Alabama, in response to the celergymen who had written an open letter of criticism (the Letter from Birmingham Jail)

And if America is to be a great nation this must become true.
So let freedom ring from the prodigious hilltops of New Hampshire.
Let freedom ring from the mighty mountains of New York.
Let freedom ring from the heightening Alleghenies of Pennsylvania!
Let freedom ring from the snowcapped Rockies of Colorado!
Let freedom ring from the curvaceous slopes of California!
But not only that; let freedom ring from Stone Mountain of Georgia!
Let freedom ring from Lookout Mountain of Tennessee!
Let freedom ring from every hill and molehill of Mississippi. From every mountainside, let freedom ring.
And when this happens, when we allow freedom to ring, when we let it ring from every village and every hamlet, from every state and every city, we will be able to speed up that day when all of God’s children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual, “Free at last! free at last! thank God Almighty, we are free at last!”
Martin Luther King, Jr., at the March on Washington for Jobs and Freedom, August 28, 1963

What binds their stories together was the back-against-the-wall, reluctant yet hopeful search for something better, any place but where they were.  They did what human beings looking for freedom, throughout history, have often done.
They left. 
In the end, it could be said that the common denominator for leaving was the desire to be free, like the Declaration of Independence said, free to try out for most any job they pleased, play checkers with whomever they chose, sit where they wished on the streetcar, watch their children walk across a stage for the degree most of them didn’t have the chance to get.  They left to pursue some version of happiness, whether they achieved it or not.  It was a seemingly simple thing that the majority of Americans could take for granted but that the migrants and their forebears never had a right to in the world they had fled. 
Isabel Wilkerson, The Warmth of Other Suns:  The Epic Story of America’s Great Migration