Historically speaking I've mostly matched characters based on the situations that brought them together <X3 Although personally I find compatibility the most important, factoring in that they're available for each other on principal and emotionally <X3
Compatibility, any relationship is based on chemistry, they can be be exactly the same and have the same ideals yet hate each other and be bad for one another all relationships need some variety and so do individuals, people and relationships usually gravitate towards people with similar tastes but are generally different with anything else. This makes characters grow as they are challenged by the relationship to be out of their comfort zones
Thats exactly what I said though they likes some similar things BUT everything else is different also that is how ALL our couples are, you should know this I like mismatched couples. It tests their boundaries and makes them more developed
Compatibility aside, I like to ask the question of: would they make a happy, healthy couple? You could pair up two characters that would find each other attractive and have similar interests, but at the heart of the relationship they can still have problems or prove incompatible.