every player i've known who needed to be my one and only was unsecretly a neurotic mess just waiting for a downward spiral. i've only put up with it in one case; in general it is a red fucking flag. play with anyone you want, just don't flaunt it. (ex: "omg, new player y is my faaaavorite x ever, i could never do anything like this with other x players!!") but that's common sense.