With an example

Think of the number line as a one block street with the zero a person who lives in the middle of the block. Call him Zero. Then think of negative numbers as being addresses of neighbors equally spaced to the left of this person and positive numbers being addresses of neighbors equally spaced to the right. If Zero goes out of his house and walks three houses to the left and goes in to visit, it would be the same distance as if he walked three houses to the right and went in to visit that neighbor. So when the distance traveled from zero is the same then the positive and negative numbers cancel out. But if Zero walks four houses in the left direction that would be a longer distance than if he walked just one house to the right....