IS YOUR CAR SEARCHED EVERY TIME YOU ARE RETURNING FROM MEXICO TO THE U.S., WHAT MAKES THEM SEARCH IT?

I am planning a trip with my family for a summer and a few questions came up as we talked about the trip. What makes the border officials search your car? Is your car searched every time you enter and/or return from Mexico to the U.S.? Dont worry we're not smuggling anything! We just thought it would be something interesting to learn about. **PLEASE NO UNNECESSARY INFORMATION OR COMMENTS, IGNORANCE IS OBVIOUSLY EXTRANEOUS TO THIS TOPIC. thanks!