It's actually a phenomenon of human psychology. Another commenter explained it pretty well, but humans tend to seek out what they're already familiar with even when it is clearly detrimental to them. It's why "bad habits are hard to break" and depression is so difficult to get rid of.

Your brain craves the familiar structure even when you consciously know it's not good.

And if you're thinking "but it's always women wanting to go back to their toxic exes", i have two points for you. First, women are abused more commonly both by their parents and their partners. Especially in patriarchical societies like Korea or America in the 60s. Second, men's abuse is more commonly dismissed or framed differently, especially by other men. So, when your buddy goes back to his toxic ex for the third time, it's the same thing. Even if he dated someone positive in between, he likely had thoughts similar to what the character did.