I wouldn't know, perhaps it's because Nanjo is so attentive and caring towards Sunhwa. It seems that deep down, Makyeon desires to be loved and chosen by someone. However, we must not forget that Makyeon also manipulated Nanjoo and made him make choices he didn't want. So Makyeon himself is not a saint either. Every individual, after all, possesses both strengths and weaknesses. All 3 are flawed, and we have only seen things through Makyeon's point of view, which, of course, is very distorted.

I think that the official name (borderline) is describing Mokyeon's mental health. It would explain his behavior.
Borderline personality disorder (BPD) is a mental health condition characterized by severe instability in emotions, relationships, self-image, and behavior. People with BPD often experience intense mood swings, struggle with a chronic fear of abandonment, and engage in impulsive or self-damaging actions.
Extreme fears of abandonment: Going to frantic lengths to prevent separation or rejection.
Unstable relationships: Cycling between idealizing others and suddenly devaluing them.
Fluctuating self-image: Sudden and frequent changes in goals, values, and self-esteem.
Impulsive behaviors: Acting out in potentially self-damaging ways like reckless driving, binge eating, or reckless spending.
Self-injury or suicidal behavior: Engaging in self-mutilation or making suicidal threats.
Emotional instability: Intense episodic feelings of anxiety, irritability, or sadness that last from a few hours to a few days.Chronic feelings of emptiness: A persistent sense of having no purpose or feeling bored.
Intense anger: Difficulty controlling anger or experiencing extreme, inappropriate rage.
Dissociation or paranoia: Stress-induced, fleeting feelings of being detached from oneself or suspicious of others' motives.