Why do I get so defensive?

Whenever someone points out one of my weaknesses, I get very defensive. I try to rid myself of the imperfection and have found myself denying what isn't completely untrue. Why is it that I get so much more defensive than many of those around me?
I guess growing up, I've been pretty obedient. I didn't do many things wrong intentionally, or merely to upset my parents. Trying my best is what I usually try to do. When I feel like I'm doing good, and someone says otherwise, even if it's true, I get defensive. At my house, my family members have learned that sometimes, it's easier to not mention my faults at all, or else it get quite upset. I wish this wasn't the case. I want to figure out why it is that I get offended so easily. I guess, part of it is due to the fact that my parents have always made me feel like I'm a good person. When something other than that opinion is brought up, I'm unused to it. I don't like being told that what I say is wrong. I guess it goes the same for my actions; I don't like being told that what I'm doing is wrong either. My defensiveness likely stems partially from my prone-to-offense nature.
As each of my friends and family-members know, I am a very very very sensative individual. This isn't entirely by choice. When something of the smallest degree happens to me, the tears can start a'flowin' from nowhere. Oftentimes, I'm not even upset, and just because something happens, I'll start crying. This can be very frustrating, especially when someone's telling me that I'm wrong. The worst, however, is when someone tells me that I'm getting too defensive. When they says this, it's pointing out one of my faults, and causes me to get even more defesive, and I often start crying at this point (if I'm not already). Sometimes, I think that my sensitivity is mistaken for defesiveness. However, this trait brings out my defensiveness, proving it even more. It's quite frustrating, actually.
Finally, I get defensive when what's being said about me is true, but I don't want it to be. I get so upset when people focus on any of my negative qualities, that I want to shun and deny them. What an unfortunate thing to be unable to face my flaws like a normal human being. I have to pretend like they're not there, subconscioiusly telling myself that I'll work on those specific things very well in the future. However, I don't like to be wrong, and denying my flaws or potential mistakes creates that defensiveness, once again.
I think I've discovered where my defensiveness stems from. I'm too offended, sensative and denying when it comes to my negative traits. How ironic, that these things add up to create yet another flaw, that being defensiveness itself. While I'm able to focus on my positive qualities, I do know that there are many things that I need to work on (even if I may seem to deny them). I need to work, very much, on my defensiveness so that I can able myself to conquer other flaws to come without offense, defense, tears or denial.