There are many people, places, times, action's taken, and reasons for said action's being taken. These are fundamentals of human life. The most important thing to remember about people is that in order to survive they must have food, water, and air to breathe. These are mandatory to live and y'all may ask why is this important, does this writer take us for idiot's. Absolutely not, with the complexities we focus so much attention on this may be overlooked.
            When people migrate to another place these are the main reasons.
                       - Famine
                       - Can't provide for their basic needs
                       - War
                       - Persecution

Example....

The Great Hunger in Ireland triggered by the failure of the potato crop in 1845 led to the single greatest loss of life in Europe. While over a million Irish starved to death and another million were forced to emigrate, food was being forcibly exported from the Island by its foreign rulers - food sufficient to feed the people several times over. This period represented the blackest days of Irish history.

Photo and Caption courtesy of

Dan Beards