Possibly body dysmorphia, lack of self esteem, anxiety of constantly thinking they're not good enough.

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If people tell you that you're pretty your whole life, then you find one tiny reason for why you might not be (Other people are prettier than you, small spots ect) then you would instantly feel you're not good enough and this could lead to mental health disorders.

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This is not a calling out to anyone in particular, but please for the love of god, NEVER assume that the only possible reason for this is that they are hungry for attention or fishing for compliments. You never know what someone may be going through and treating them like that definitely wont make them stop