Many people assert themselves with their "lover". "MY" girlfriend, "MY" woman. It express the possession over one thing.
Girl and women are person, not things... And even if they "love you", I don't think it's enough to say that "you possess" them.
Most of the NTR drama come from it: you think that "x" person is yours, so if she decide to "leave you" you go "OH MY GOD! That SON of bitch STEALED my WOMAN"... No he didn't LOL, she just decided to do things by herself. With this, came the second thing: you know, woman and girls have an "intellect", a "brain". They are more than able to "chose" by themselves what they want. If they want to stick with you, they'll just stick with you... If they'll change their mind, they'll chose to do other things.
People are insecure, that's why they try to find a "landmark" in a girl or in a guy... And whey they lose this "landmark" they go in pieces. It doesn't hurt cause NTR is bad, it's hurt cause people are sensible to those kind of things.
Same goes for the Netori stuff: "HEY! See? I'm FUCKING YOUR girl in front of you! See what she say? I'm BETTER than you, I'm STRONGER and more BEAUTIFUL". Same stuff from another prospective