When Dean found you, he immediately took you in, no questions asked. So now, when you always tell him and Sam goodnight before going to bed, constantly wake up to check on them, and never trust anything until they take it, he couldn't help but wonder where you had been before he found you, what had happened to make you act like this. So now, when he, you, and Sam have to go through your best and worst memories on a case to get you to wake up, he starts to understand why you are the way you are. Yet even that doesn't help how sad it makes him to see what's happened to you at such a young age.