Why do we dislike vanity?

Closely associated with pride, one of the seven deadly sins, vanity is something often seen as a negative thing. After the reappearance of the ever vain Samantha Brick in the news the notion of vanity has resurfaced. Many comment on how Brick's confidence in herself is just a toxic sense of vanity but what I want to consider here is why we're so against vanity itself.

 Surely we all know that person whose Facebook or blog consists heavily of photos of them in various different outfits and poses. 'How vain' we may think. Yes? So? Simple vanity itself doesn't seem to harm anyone. One possible reason is because in a vain person we see someone who is confident in themselves in ways we cannot be confident and therein lies some form of jealousy.
In Britain traits like modesty and humility often seem sociocultural expectations of character. We are often apprehensive to talk about our achievements and find the process of CV writing and job applications a little daunting.

In a time where notions of beauty are regulated, controlled and broadcast so heavily by the media and most women live their lives totally unsatisfied with their appearance as a result. In the articles I read about Samantha Brick one term often used to repeatedly describe her was 'self-satisfied' which prompted me to question what is so inherently wrong with being satisfied with ourselves? I'm not saying I agree with all of the articles and views Samantha Brick has broadcast in recent years. I'm simply questioning why we're so quick to point a finger and scream at vanity. Have we so heavily internalised the media-given images of what 'beauty' is meant to be that we then attack anyone who refuses to feel totally shitty about themselves?

Why should beauty be the sole privilege of the beholder? I've been overweight to some degree since I was at least 15. Even if it was simply a few pounds or when I was about a size 18 I was bigger than the typical healthy person should be. The strange thing is that it is only once I reached the age of 19, around a size 16 or 18, that I felt more beautiful than I ever had in my life. I'm still pretty curvy, I have piercings and a haircut that isn't seen as typically feminine but I feel beautiful. Does the borderline between my current state and 'vanity' lie in whether or not I speak about how I view myself? I simply cannot fathom why we criticise those who see beauty in themselves when we're constantly being told otherwise. Surely it takes more to ignore the messages we're constantly being subjected to and to be comfortable in our skin.