“Opposites attract” primarily refers to the immune system. People are naturally attracted to people with an opposite immune system from theirs. Which is why some people can smell better to you than others. Opposite immune systems will smell better and those that smell horrendous may refer to people that have immune systems that are too similar.
Because when people with opposite immune systems breed, their offspring will have a stronger immunity.

When it comes to opposite personality traits, as time passes a greater rift can start to divide them