You know those people who drive you crazy? Yeah? Well, you need to have some of them in your book. You see, variety is key to a good story. It also helps set up personality conflicts. Know what I mean? Think Odd Couple. Oy, I just dated myself. But truly, Felix Unger and his polar opposite are the perfect example of a way to generate conflict using characterization.
Just like marriage is the blending of two personalities, fictional characters (especially in romance) should also discover ways to reconcile their differences. To learn the give and take of seeing things through someone else's eyes. Eventually, in your writing, if you work at it enough, the swapping of personality strengths and weaknesses between your characters will begin to make them multi-dimensional. Real.
Opposites *do* attract. Iron sharpeneth iron, and all that. In a romance particularly, and really in any genre, though to a lesser extent, these conflicts of personality can be an integral part of your story arc and produce many hurdles for the protaganist to overcome.
Think of the angst of someone who wants to learn to "get along" with someone else but doesn't know exactly how to go about it. What happens to this person? One of two things. Either he learns the process, or he gives up.
Characters who resist any chance at getting to know others (anti-social) can also be an interesting twist. Thrillers employ this type of character trait all the time in their villains.
What is one of the most interesting play of personalities that you've read in fiction?