Why does donald trump want to build a wall on the border for mexico?

To keep illegal immigrants out, and to make people mad and miserable.

He has many reasons for wanting to build a wall along the southern part of the united states the main reason he wants to do this is to keep out anyone who wants to travel into the country without paperwork which in turn should bring down the amount of drugs smuggled into the U.S.  

Andean mountains amazon basinatacama desert brazilian highlands