Whatever the secret to a successful relationship is, the Prince and Princess of Wales seem to know it: they met at the University of St. Andrews in 2001 and developed their friendship into a romantic relationship around 2002 or 2003. That means the couple has been together for over 20 years (except for a couple of months in 2007, which we never talk about), and since they are both 41, they have been together for half their lives.

Hello magazine reports that one of the secrets to their romantic success may lie in a personality trait they share: they are both very competitive. Competitiveness may not be the first trait that comes to mind when thinking about the formula for a successful marriage, but it works for both of them.

"Prince William and Princess Kate always seem to enjoy performing together in public. From spinning to sailing to running, the Prince and Princess of Wales never seem happier than when they are engaged in some healthy competition." In fact, many of their joint performances tend to lean toward the scenario of the two competing against each other in some activity.

Jade Thomas, psychotherapist and founder of Luxe Psychology Practice, said that this competitive spirit may be the key to the success of their relationship. "Competition may bring considerable excitement, motivation, and drive to their relationship." Two people who are competitive are likely to be compatible because they share similar values and are both equally motivated."

Psychotherapist Dana Moinian agrees. She says, "Joint competition is encouraging for both members of the couple. Competitive people generally like to set benchmarks higher and higher, so when two people in a relationship have the same mindset, (it) can be beneficial to both of them in terms of progress."

Nevertheless, both Thomas and Moinian agree that competitiveness between partners needs to be reined in to avoid it becoming harmful. Thomas says, "It is important to be aware of what competition is in the relationship." For example, if both parties are constantly competing to outdo each other, it can lead to conflict and low self-esteem, which can be detrimental to the relationship."