Why are things different when we cross the U.S.-Mexico border?

I’m not sure why things are so different when you cross the border… I mean it’s just another place with people. I know people are crossing the border because they want a better life for their family and for themselves. But I know there is a difference when you cross the border. When I look out the window after crossing the border I see a lot more traffic than on the U.S. side of the border. I also see a really long line to cross to the U.S. There is also more trash on the streets and it doesn’t look as nice as it could. There are also people who have to run across the street because some cars do not stop, and there are not many cross walks. (I know how this feels.) I also see more less fortunate people and some houses.. are not even houses at all. It also smells very badly in some areas because of drains and other stuff. There also seems to be a “rich” side of Mexico and a “poor” side, and you can tell the difference by how the area looks. I do not think places that are so near each other should be that different.