I have a really low tollerence to being criticised - I think just because I was never good enough (and am still not) growing up. I know that some criticism is critical, and can be vital, but that doesn't stop me from feeling attacked by it.
It's just one of my 'things'. I have so much trouble, and make such an effort, convincing myself that I'm not worthless, and that I am 'good enough', that when someone tells me I'm not - I don't deal with it very well.
Of course I hide it, and no one realises how much it hurts, but it's there.