I never understood....

...how some people can be so happy, and having so much fun when they watch a girl they love dating some other guy, in front of them. Especially when they told them that they loved them. I saw this video of an indian guy and his gf. They were in a club, and the girl told him that she's dating some other guy. Then she tells you to watch me, go ahead and watch me. Later he was dancing together with them, next to them. I just don't get it. She's not my love any more, I don't want to see her at all.