You can probably imagine that it would take a very strong reason for a family to pack what they could and move to another country. They might have to leave all of their favorite things behind. They might have to learn a new language. They might have to learn a whole new way of living. An Eskimo would have to learn to adapt to warm weather if he moved to Hawaii. Someone from China might have a hard time understanding what people were saying if he or she moved to Brazil. It could be very hard to adjust.
A sociologist is a person who studies the way people live. Some sociologists have come up with theories about why people immigrate to a new country. They describe the reasons as "push and pull" factors. Look at it this way. What happens if you and your friend try to open a door at the same time from opposite sides? You might be on the side that must be pushed to get the door open. Your friend might be on the side that needs to be pulled.
A very good example of push and pull factors and their effect on immigration can be found in the job market. Let's say that you live in a very poor country. There are very few jobs available to you that can give you the money you need to support your family. The lack of jobs pushes you to leave your home country. Let's imagine that the country next to yours has a lot of jobs to offer. The chance to get one of those much needed jobs pulls you to move to that country.