With regards to writers' ability to take criticism (or lack thereof), I don't think attachment to their story is the only factor or even the most important factor for many as to why criticism is hard to take. Someone's own beliefs about themselves, their ability and how skills develops makes a big difference, and leads to writers putting themselves under a lot of pressure (usually subconsciously) and that makes it very hard to cope with criticism.
For example, if someone believes that writing ability is mostly down to inborn talent that you've either got or you haven't got, and that writers who have that inborn talent produce first drafts that are scintillating and everyone immediately recognises their talent from them, when their writing is criticised, it's going to be interpreted (probably at a subconscious level) as "you're not good enough" and "you don't have what it takes" and "you have no talent". This then leads to massive loss of confidence and/or rejecting the critiques completely wrong, rather than taking the criticism on board and using it to improve your writing.
Similarly, the believe that anything less than perfection is not good enough is not conducive to taking criticism well. And there are probably various other thoughts/beliefs that cause writers to put themselves under way too much pressure and therefore react badly to critique.
Just as learning to drive a car becomes automatic after a while (i.e. you can drive without thinking about driving), learned patterns of thinking and beliefs like this end up being processed automatically (generally they're learned in childhood and are processed entirely automatically in adulthood) and you're not conscious of them. So just because the negative thinking patterns aren't present in your conscious thoughts, it doesn't mean they're not there and affecting how you feel and react when your writing gets critiqued.
The fact of the matter is that no-one but no-one gives a flying fuck how good someone's first draft is or how much time or how many revisions it took to get from that shaky first draft to the finished product. And no-one gives a damn if you thought you'd got a finished, polished, story then you submitted it for critique and people found stuff wrong with it that took you ages to fix. They don't care if you submitted it for critique twenty times and people still found stuff you could improve on. What matters is the finished product, and a finished product that's been through the critique process is going to be miles better and have a lot more chance of being saleable than one that never went through the critique process.
If any of the above rings true, you can reprogram your subconscious beliefs, though it takes time, by repeating the new beliefs on a regular basis (many times a day) - having them written where you can read them several times a day also helps - and practicing being critiqued. It might hurt a lot the first time you do it but it gets better after a while and in the end you should get to where critique is (on the whole) a very positive, motivating experience that leaves you eager to put useful feedback into your story to make it even better.