Why we run after emotionally unavailable people?

You have probably heard many times to follow your heart, but what if your heart is constantly running after emotionally unavailable people?

That guy is probably in an unhappy relationship, or always working, or is an addict, or he doesn’t have any explanation for being like that, he is “just” emotionally unavailable. I said “the guy” in the previous sentence, but it works vice versa. This issue touches both sexes.