It wasn't a good marriage. They married because of an attraction to something new and exotic. Sometimes, attraction happens when you least expect it. He was a prince of a distant country, and shew as a woman full of life, who was very appealing, who came from a very different culture. When she comes to Dorne, she finds that there are customs that are different from those of Norvos, especially regarding the fostering away of children to others. This wasn't a political marriage, nor a magical one, it was simply an example of human nature. Sometimes relationships start on a good foot: you become acquainted, there's a great sexual attraction, you establish a relationship, you marry... and then in four or five years you realize that you don't really have anything in common, that at best you've made a mistake and are in a situation that doesn't have any easy solution in a society such as that of the Seven Kingdoms, where divorce simply isn't common. This is an example that it's not only marriages of convenience that fail, but even the marriages for love can fail.
Sometimes the marriages of convenience in the Seven Kingdoms come out well and those that are for love don't. Sometimes a couple loves one another, and then at some point they don't. There are marriages that also develop out of nothing more than lust (laughs). There's no guarantee that things will go well and the consequence of this is that disappointments develop and you end up estranged, each person going their own way. There's some bitterness from Mellario about this, because as Prince of Dorne, Doran has been able to stay with his children and she has had to leave them.