Simply because your face is ''the wrong way round''. 90% of people hate taking pictures of themselves, and they think they are the least photogenic in their families, most of these people prefer their pictures when they are "flipped" on the computer, we live our lives looking at our faces through the mirror, we are used to seeing them in that way, it just does not sound right when we reverse it.
No one has a completely identical face. Most people separate their hair on one side instead of the other. They have a slightly larger eye, a curved eyebrow, a more straight nose, a scar or features on one side than the other. They laugh at one side of their mouth more than the other, etc.
If your nose is tilted 2 mm to the left, then when your image is in the opposite direction it will appear to tilt 4 mm to the right in terms of what you expect it to be. Considering all of this, when you see your face contrary to what you expect, you will see and see yourself at the same time, Which makes you feel uncomfortable, For example, this picture of one of the world's most famous faces Mona Lisa (Leonardo da Vinci). Which one do you prefer - the one on the left or right?
Almost everyone will prefer the left picture because most of us are satisfied with what seems familiar.
So when you look at a family or a group photo, everyone else looks as you expect them to (-the way you always see them), but that does not apply to you, Your face is in ''the wrong way'' not what you are expecting, therefore you think that you're very unphotogenic !
So every time you tell someone, ''you look great, but I look awful in this''
They won't understand'' because to them you look fine and they are the ones who look awful!
Here's a small experiment for you, take a selfie '' that you think you look awful in it'', hold it up in the mirror and you will think it's much better !