Sometimes the people who are supposed to care for you don't, and it can be a real shock when the obvious is pointed out to you. I remember my mother saying she had abused and neglected me, I had never used the words abuse or neglect on her because it just never occurred to me. She probably abused me more than anyone (minus her boyfriend of 4 years), yet as much as I knew she had done all these things and there was a lot I resented her for, I was so used to it that I didn't realise the extent of it.
But once it's been pointed out to you, a lot of stuff can make more sense to you, you have an answer for your resentment and you an analyse at lot of your feelings. But it can really hit you too like a bombshell. How could you have NOT noticed? It was right there! Happening to you! And it had to be POINTED out to you?
I guess it can also explain why a lot of people stay in abusive relationships, everyone on the outside can see exactly what's happening and can't understand why somebody would stay in a situation like that, but the abusee has no idea what's happening to them.
But it's not uncommon for people not to realise until someone tells them, abuse doesn't just HAPPEN, it's something that slowly builds.