Ooh! Ooh! Ooh!
Random Theory!
So you're the person who is hired as a nightguard, blah blah blah. Big deal. You've never heard of Freddy Fazbear's Pizza, you'd assume it's a simple, child friendly place, you've never been there to witness the bite of 87...
...Or have you?
I like to believe that you are the child who was bitten by whatever animatronic (presumably Foxy) was responsible. The first red flag is the fact that you don't get backstory. This could possibly be a very vague hint that because your frontal lobe is missing, you don't remember the incident. However, this is very weak evidence for me to use, as it could simply be that Scott didn't want to tell you what you were up against before you got the phonecalls, as well as the fact that Wikipedia states nothing about major memory loss.
Now here's where I'm gonna dig deep for information, and I mean deep.
According to Wikipedia, "The frontal lobe contains most of the dopamine-sensitive neurons in the cerebral cortex. The dopamine system is associated with reward, attention, short-term memory tasks, planning, and motivation."
Keyword: Most. Some of these areas seem ok, but others don't, specifically attention and short term memory tasks. The frontal lobe contains most of the dopamine-sensitive neurons, but not all of them. Where is the red flag here?
The phone calls. In the first phone call, the phone guy says something about how the Animatronics will try to stuff you in a wire-filled suit. Other than that, it's a regular run of the mill pizzeria. Your frontal lobe-less self would probably not listen to most of the phonecall, and not register the costume warning. This also feeds into short-term memory tasks. Even if you did register the warning, chances are, you'd forget about it. And considering that the frontal lobe has most of the dopamine-sensitive neurons, you might even forget the terrifying faces you saw through the camera and the door, which is why you would keep coming back.
Still not convinced? Get this.
"Memory impairment is another common effect associated with frontal lobe injuries"
Every night, every scare, this is an effect of the bite of 87. You keep coming back because you don't remember the danger.
The frontal lobe is also responsible for Schizophrenia.
(Quotes from Wikipedia)
Common symptoms include false beliefs, unclear or confused thinking, auditory hallucinations, reduced social engagement and emotional expression, and inactivity.
The average life expectancy of people with the disorder is ten to twenty five years less than the average life expectancy.[6] This is the result of increased physical health problems and a higher suicide rate (about 5%).[4][7]
Higher. Suicide. Rate.
It is possible that FNAF isn't even real. You made up the pizzeria after a brain injury of some sort, or even recalled bits and pieces and filled in the gaps. Another possibility is that you're in the pizzeria, but in reality, the animatronics are actually shut down, and don't wander during the night.
Whatever you interpret it as, one thing is the same.
You are hallucinating, both visually, and auditory. It would explain why the animatronic's eyes sometimes go black with whites in the center. It's hallucinations.
When you die, it's not the animatronics, it's yourself. You committed suicide.
If anything's wrong in my theory, let me know!