You can really see the progression of their relationship in that flashback. On the one hand, Flint and Alfine did work decently together, and did have some consideration for each other's well-being. On the other hand, you can see hints of how this relationship became so toxic (the belittling, Flint getting pummeled while Alfine tries to 'power up', how Alfine handles stress and how that affects her relationships with others). It's probably not a surprise that Flint decided to leave everything behind- who'd want to be reminded of the relationship that made them miserable?
Now to see if that Noelia girl can keep herself from becoming like Alfine. I don't think she will, but if she does, it would seem like Flint attracts a certain type of women, huh?