Migration has always existed in our world and it always happened for similar reasons. Through out the history there have not been just normal migrations, which means a small number of people, some of them occurred with hundreds of thousands or even millions of people running away from their own country. Some examples are: the great migration at the beginning of the twentieth century and the big migration from the nazi regime. The migrants are classified in various groups, the main ones are "Economic migrants", those who want to abandon their country of origin to find a better payed job or to find a better way of living and "Refugees", those who run away from their country due to war or persecutions.
The economic situation of a certain country can force a lot of people to migrate. Most countries offer better salaries for the exact same job and this situation is creating a salary gap that is getting larger and larger as the time passes.
Some countries are very supceptible to climate changes due to their life styles. A sudden or long-term change in the enviroment can set most of the primary sector in a crisis, compromising the well-being or secure livelihood of the local population.
Persecution, discrimination, harassment and even torture to punish those who doesn't agree with the government are a few of the many political reasons to leave your country behind. People who comes from these realities usually are also fleeing from war, so they look for countries which can offer a better array of rights to get a better life.