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It's from fear sprung out of the inability to clearly identify and interpret something and from that know how to react to it.
There's something skewed with what you see, something deviating from your mental model of what a face (in this example) looks like. That makes us perceive it as something unknown, and what's unknown is also perceived as unpredictable and induces an uneasiness and discomfort.
The reason why this is unpleasant:
But this isn't:
Is because the latter is clearly an illustration and can be subconsciously dismissed as something static/dead, and by that predictable and nonthreatening. In the first image we have a hard time identifying what it is we see, whether it's something that's alive or if it's dead. This makes us uncomfortable since we don't know how we should react to it, even if it's just an image.
Being confronted by it in real life would without a doubt be very disturbing since then there is no escape, it's very real, very unknown and therefore very unpredictable.
There is a similar face possible in reality. It is a victim of burning who later received face transplantation. Looking at it is shocking. Maybe it leads to imagining the suffering? I am not posting it here—it is not pretty—but see this link.
Edit: Actually, my example seems to be covered by uncanny valley: "Examples can be found in the fields of ... and in medical fields such as burn reconstruction ... and plastic surgery".