It’s pretty simple. We get natural ‘heroin’ when we are attracted to a woman. we get more when she pays attention to us. and we get a lot more of it when we have sex with her. we get accustomed to it and agitated by its loss. And then we develop a tolerance for it.
The same is true for negative experiences. It takes about three to five years to ‘unlearn’ a trauma. It takes the same to unlearn a ‘dose of pleasure’.
I am not sure why this is hard to understand.
Even if you really like someone, without all sorts of incentives to carry you from year 7-20 (at which point everyone stays together – if for no other reason than transaction costs, and marketability) then it just ‘wears off’ and you want another ‘fix’ of heroin.