this is because a mirror shows a reverse view of you, a photo doesn't - you're used to the reverse view and think of that as the good-looking you, so when you see the ACTUAL you, well you don't like it. In contrast, if someone else stands behind you and looks at you in the mirror, they will think you look worse there then in pictures, cause they are used to the picture version of you.
I know this girl, who by all means, is very unattractive. But is very photogenic. She gets like 200+ likes on facebook for all of her photos, and looks like a borderline model in them.
yh same, somedays I think "eew I am sooo ugly, I should just cut my face off and put it out of its misery" and others I think "wow, I'm pretty cute :)"
Talk about mysteries of the earth huh?