gel blend mesh

Why do certain people "gel"? Why do birds of a feather flock together? What makes you like me and me like you? Why do we desire friendship? It's always interesting to me to find couples that don't need friends outside of their relationship. They share just each others' interest for the other and need no other personalities to add to their stimuli. Of course there are many types of couples....couples where one is gregarious and the other member isn't....so one half has a zillion friends and the other is silently begging to leave the party and get back to that sofa, that solitude, that cozy of the you and me...while the other half is encased in joy with no end until of course, it's time to end the festivity and go home. Some couples blend so well they begin to look like one another. And others mesh so seamlessly, you can't tell where one part of the couple begins nor where the other takes over.
I imagine there is a way to "sort" people and categorize them into matching pairs or groups. A new app for Face book to tell you if you should befriend Johanna James because she likes cool jazz just like you do. If I like cool jazz and you do too, will that be enough for us to enjoy each others' company? If so, for how long? And in what capacity? These sorting tools must be what makes dating websites and services successful. How do they decide what to ask on that questionnaire? I like salt on my pizza, socks on in bed, white toothpaste and sunny afternoons. I doubt these are listed there and if you like pina~ coladas, well, you get the idea.