Not least because the image is "flipped" in a mirror, so to make it a true representation of what others see we need to reverse the image (and that always looks "wrong" if you try it because it's not how we're used to seeing ourselves).
But even what other people see when they look at us is distorted based on what they expect to see, and what their personal filters and bias are.
Compliments, or indeed any kind of feedback from other people is nonetheless incredibly valuable. Not because we can rely upon its accuracy - or not- but because it helps us see things about ourselves we may be blind to. And all the bits of data help us to montage a more or less accurate representation of ourselves.
Unless our view of ourselves is negative and "protects" us from compliments - and in the worst-case scenario can protect us from love.
When, for lots of complex reasons, none of them positive, someone holds a very strongly negative view of themselves it's almost like that erects a force field around them, and the views of others - positive as they may be - bounce off. They never make it in and so the person can be unmoved by them. Their view of themselves remains unaltered.
Like most things, this can be changed by repeated exposure, and practice.
Most importantly though, you don't have to believe it. You don't have to believe that you are a good person, smart, attractive or funny.
You just have to work on accepting that someone else thinks you are - even if you also have to think they're a bit mad for thinking such a ludicrous thing!
Because despite what you might feel, other people are the solution. The love and care of others are how our earliest ideas of ourselves are formed, and it's also how we change, shape and form our view of ourselves and our abilities as adults.
And over time, this works. Mainly because spending time around people who think we're pretty fab is a good recipe for life. And eventually, we may even come to believe it ourselves - even if that's always balanced by a healthy dose of humble doubt.
