I dont really see that. Nic and Alexandra loved each other, and were close and that must have helped them through the bad days. It wasnt' a good thing that they were both pretty stupid and that Alexandra didn't get on with most people, from the Imp Family to the court to the common people.
But with modern day royals I dont really see how one can tell who is in love or isn't. Royals have to act, in public. They may have started out in love in some cases and as time passes teh love has gone cooler or dull.. but they are still going to be seen as enjoying each other's company in public. Fergie and Andy were in love I think at the beginning and were always looking happy and fond and with a lot of PDA but within a few years, they were divorcing. It was probalby true at first, that they loved each other but it didn't last long.
with other royals I am a bit doubtful as to how much in love they are. After all, in most cases, the wife is from a lower rank and is problaby not as rich.. so that has to be a bit tempting. As against that, of course a woman can marry a rich man who DOESNT have the burden of royal life and public appearances...