Thanks.

The logic behind the pensieve is simple; if a child thinks they deserved to be abused (which, tragically, often happens) one shows them another child being abused, which will force the child to accept that, in this situation, the abuser is in the wrong, not the abused. Once this point is established, it is easier to convince the child that they were merely an innocent victim and that the abuser them was wholly in the wrong. It may seem a little harsh but, if Harry couldn't accept that he did not deserve to be abused, he would never recover. (Also, 1. Harry saw Slytherin's 'happy ever after' first, 2. the more violent memories were cut to a bare minimum and 3. he saw Slytherin being rescued.)