A possible scientific explanation for this is that it helps add diversity to the gene pool and to the relationship, because the two contrasting styles of thought of the two individuals can help solve a problem, so long as they listen to one another's views rather than just dismissing them - and you'd hope they wouldn't dismiss the views of their partner in a relationship!

Although this therefore helps in some instances, things can break down if the differences become too wide and begin to seep into everyday life, such as if the couple begin to argue about trivial things that they disagree on. I've seen this happen, albeit to a small extent, and have reason to believe it has caused the break-up of other people I know. After the initial attraction of being opposite, it can sometimes be a bit tough to face the rest of your life with someone so different.