He the black hair guy (BHG) stole the white hair guy's (WHG) lovers because he didn't want anyone close to the WHG, plus like the BHG confess in their teen years, he can't help but crave/loved everything WHG had/love. But like the WHG said to the young detective, anything BHG gets, he (BHG) eventually leave because he (BHG) gets bored of them. So WHG would run away of BHG, so BHG would chase him and therefore keeping a relationship with BHG without actually beginning anything romantic, since the white hair guy (WHG) knew that the black hair guy's (BHG) love theory is " in love if you being it, then it will end". So when white hair guy confess, he also ended their relationship since by confessing his love, he (WHG) began something and by doing so he (WHG) stop running (stop the chase) so their love game (both BHG and WHG) was doom to end, in accordance to the black hair guy's love theory.
I don't get it. I love you but I slept with all your previous lovers and of course I confess when you are on the edge of dying 50 years after and because I was dumped I gave up easily and date a younger man. There's really something wrong with that black hair man.