You ask yourself who you see yourself further down the road with. Who do you see yourself growing old with? For me, the fact that my SO not only accepted my 'dark side' but even loved me because of it, sealed the deal. I've rarely met someone who rarely misunderstands what I'm saying and in case it does happen, will always give me the benefit of the doubt. I wasn't about to let that get away.
Where I might have a great understanding, and even an intense connection with others, to the point where they are very much datable and interesting that way, it's pales in comparison of what I have with him.
I flirt a little with most, as an icebreaker, but it's the reaction I get from the other person that determines if I up the ante. It gives me a chance to get to know them and see if they are in fact interesting as a potential mate.