It's also a scientific term related to how magnets work. Try putting two magnets together. You have to use opposite sides together. If you flip one of them around and try to put them together that way they will be propelled apart.
I prefer to use the term "complimentary" as oppose to opposite. Two people with similar personalities might find each other boring because they always know what to expect. With opposing personalities, you fill in the holes with each other and "compliment" each other. It's a good thing.