Royal weddings are often arranged. Royals want someone of a similar social and economic class for their heir. Historically, royal arranged marriages have been a way to gain more territory or strengthen the power of the monarchy.
It looks like these outdated customs are slowly being done away with though. Just look at Prince William and Kate Middleton--a prince and a commoner! And they seem quite happy.
True love was also the motivation for Crown Prince Harald of Norway. He had been dating Sonja Haraldsen for nine years when he told his parents that he would marry her or no one at all. Their relationship had been kept fairly secret because Sonja was the daughter of a cloth merchant, and there was opposition to her non-royal status.
Prince Harald finally put his foot down and told his parents that he was either going to marry Sonja or he would marry no one at all. As he was the only heir to the throne, this would have ended the family's royal rule.
So his parent's gave in and allowed him to marry his love. The couple wed in 1968 and Sonja went on to work with many charitable organizations as the new Queen of Norway. Sounds like the stuff fairy-tales are made of.