Bro, if someone said this to, you fucked up. As in reference to the Netflix Original 13 Reasons Why, Hannah Baker made 13 tapes of reasons on why she killed herself. If someone said “You’re my 13th reason why” You fucked up and the person is saying you are a reason they killed themselves.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.