you were a insomniac, a schizophrenic insomniac that has dissociative identity disorder. Which means you have atleast one other alter.. and that happens to be Tyler durden, so at night during periods of insomnia, Tyler takes your place. And for some odd reason, you see Tyler as a person during the day.

you just got off the phone with someone who was calling you Tyler, and since the only Tyler was your friend (aka your alter) you confronted him

"Sit."

Tyler sighs, and as you sit he says the words "good boy" under his breath

"You know why they were calling you Tyler."